🩹 Workaround invalid Gmail FLAGS response

[nevans]

Gmail allows (or allowed) users to create flags containing some invalid atom-specials chars, such as "]" or SP. Ultimately, this isn't unambiguously parsable. But we can workaround it.

Prior to #212, the ResponseParser simply grabbed everything inside the parentheses and then scanned through with a very liberal "flag" regexp. This PR attempts to parse it strictly at first and if that fails falls back to the "quirks mode" parser, similar to the original.

Fixes #241.

This also uses delete_prefix! to avoid another string allocation when converting flag strings into symbols.

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Benchmark results:

Calculating -------------------------------------
                                                     v0.4.7-6-g3f88747    0.2.3    0.3.7    0.4.2    0.4.7
                         fetch_msg_att_flags_and_uid           53.343k  33.978k  33.562k  34.063k  52.615k i/s - 162.079k times in 3.038415s 4.770048s 4.829303s 4.758267s 3.080481s
                       flags_with_various_flag_types           95.497k  73.549k  73.802k  73.777k  93.892k i/s - 289.950k times in 3.036226s 3.942294s 3.928770s 3.930078s 3.088128s
imap.gmail.com allows invalid atom-specials in flags           32.155k  41.518k  38.704k  37.842k    ERROR i/s - 100.595k times in 3.128400s 2.422950s 2.599085s 2.658261s 0.000000s
              list_with_various_flag_capitalizations           42.105k  48.496k  45.108k  31.679k  39.903k i/s - 118.998k times in 2.826218s 2.453753s 2.638075s 3.756420s 2.982145s
      resp_code_PERMANENTFLAGS_rfc3501_6.3.1_example           56.608k  44.082k  44.029k  46.168k  56.743k i/s - 184.142k times in 3.252913s 4.177284s 4.182247s 3.988496s 3.245177s
      resp_code_PERMANENTFLAGS_rfc3501_6.3.2_example           62.031k  47.918k  49.941k  56.477k  63.410k i/s - 197.518k times in 3.184188s 4.122017s 3.955044s 3.497315s 3.114928s
    resp_text_PERMANENTFLAGS_with_various_flag_types           55.224k  41.582k  41.547k  46.085k  54.984k i/s - 178.322k times in 3.229081s 4.288422s 4.292034s 3.869394s 3.243159s
                rfc3501_7.2.6_FLAGS_response_example           79.586k  61.576k  62.134k  62.640k  79.194k i/s - 268.613k times in 3.375129s 4.362270s 4.323131s 4.288183s 3.391847s

Comparison:
                         fetch_msg_att_flags_and_uid
                      v0.4.7-6-g3f88747:     53343.3 i/s
                                  0.4.7:     52614.8 i/s - 1.01x  slower
                                  0.4.2:     34062.6 i/s - 1.57x  slower
                                  0.2.3:     33978.5 i/s - 1.57x  slower
                                  0.3.7:     33561.6 i/s - 1.59x  slower

                       flags_with_various_flag_types
                      v0.4.7-6-g3f88747:     95496.8 i/s
                                  0.4.7:     93891.8 i/s - 1.02x  slower
                                  0.3.7:     73801.7 i/s - 1.29x  slower
                                  0.4.2:     73777.2 i/s - 1.29x  slower
                                  0.2.3:     73548.5 i/s - 1.30x  slower

imap.gmail.com allows invalid atom-specials in flags
                                  0.2.3:     41517.6 i/s
                                  0.3.7:     38704.0 i/s - 1.07x  slower
                                  0.4.2:     37842.4 i/s - 1.10x  slower
                      v0.4.7-6-g3f88747:     32155.4 i/s - 1.29x  slower
                                  0.4.7:         0.0 i/s

              list_with_various_flag_capitalizations
                                  0.2.3:     48496.3 i/s
                                  0.3.7:     45107.9 i/s - 1.08x  slower
                v0.4.7-6-gd7bf81d-dirty:     42105.0 i/s - 1.15x  slower
                                  0.4.7:     39903.5 i/s - 1.22x  slower
                                  0.4.2:     31678.6 i/s - 1.53x  slower

      resp_code_PERMANENTFLAGS_rfc3501_6.3.1_example
                                  0.4.7:     56743.3 i/s
                      v0.4.7-6-g3f88747:     56608.3 i/s - 1.00x  slower
                                  0.4.2:     46168.3 i/s - 1.23x  slower
                                  0.2.3:     44081.8 i/s - 1.29x  slower
                                  0.3.7:     44029.4 i/s - 1.29x  slower

      resp_code_PERMANENTFLAGS_rfc3501_6.3.2_example
                                  0.4.7:     63410.1 i/s
                      v0.4.7-6-g3f88747:     62030.9 i/s - 1.02x  slower
                                  0.4.2:     56477.0 i/s - 1.12x  slower
                                  0.3.7:     49940.8 i/s - 1.27x  slower
                                  0.2.3:     47917.8 i/s - 1.32x  slower

    resp_text_PERMANENTFLAGS_with_various_flag_types
                      v0.4.7-6-g3f88747:     55223.8 i/s
                                  0.4.7:     54984.0 i/s - 1.00x  slower
                                  0.4.2:     46085.3 i/s - 1.20x  slower
                                  0.2.3:     41582.2 i/s - 1.33x  slower
                                  0.3.7:     41547.2 i/s - 1.33x  slower

                rfc3501_7.2.6_FLAGS_response_example
                      v0.4.7-6-g3f88747:     79586.0 i/s
                                  0.4.7:     79193.7 i/s - 1.00x  slower
                                  0.4.2:     62640.3 i/s - 1.27x  slower
                                  0.3.7:     62133.9 i/s - 1.28x  slower
                                  0.2.3:     61576.4 i/s - 1.29x  slower