Fix trailing commas in flow sequences

[hexane360]

This change allows the graceful handling of trailing commas in flow sequences. I've also added a couple test cases.

It relies on peek(composer.input) returning nothing in the case of a FlowSequenceEndToken which is a bit hacky, but it works well enough.

Fixes #114

[codecov]

Codecov Report

Merging #116 (1777133) into master (de8fa78) will increase coverage by 1.85%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage   84.38%   86.24%   +1.85%     
==========================================
  Files          12       12              
  Lines        1531     1534       +3     
==========================================
+ Hits         1292     1323      +31     
+ Misses        239      211      -28     

Impacted Files	Coverage Δ
src/composer.jl	91.01% <100.00%> (-0.10%)	⬇️
src/parser.jl	83.52% <100.00%> (+7.87%)	⬆️
src/scanner.jl	84.52% <0.00%> (+0.08%)	⬆️
src/buffered_input.jl	96.77% <0.00%> (+3.02%)	⬆️

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[kescobo]

This looks fine to me, but I'd love to get another set of eyes on it if possible.

Is the peek(composter.input) thing a behavior that could change, or hacky for some other reason? Either way, is it worth adding a comment mentioning why it's hacky or unintuitive for the sake of future maintainers?

[hexane360]

To be honest, the hacky part was already there, in that _parse_flow_sequence_entry could randomly return nothing. This change just handles that case appropriately. I'm not familiar enough with the codebase to know if randomly returning nothing is expected behavior for composer.input. I'd suggest adding an output type hint, but the way the code is structured makes it impossible to propagate that to the users of composer.input.

One alternative would be for _parse_flow_sequence_entry to ingest the ] token and produce a SequenceEndEvent. That might be more idiomatic.

[kescobo]

Gotcha. If the change you propose is outside the scope of this PR, could you just open an issue so that it doesn't get buried once this is merged?

I'll just leave it open for a couple of days in case anyone else wants to review it - please feel free to ping me if it hasn't been merged by the end of the week.