Preserve trailing slash in maktaba#path#Split

[dbarnett]

Also improves maktaba#path#MakeRelative to preserve trailing slash and adds a maktaba#path#StripTrailingSlash().

A little profiling since I was concerned about hotspot performance regressions…
Profile summary before this change:

FUNCTIONS SORTED ON SELF TIME
count  total (s)   self (s)  function
  874   0.070503   0.042762  maktaba#path#Join()
 3296              0.042045  maktaba#path#AsDir()
   42   0.040494   0.025003  maktaba#rtp#LeafDirs()
  658   0.021195   0.014652  maktaba#setting#ParseHandle()
   41   0.054411   0.012718  maktaba#rtp#Join()
  559   0.018049   0.012574  maktaba#value#Focus()
  610   0.056664   0.012383  maktaba#plugin#Flag()
 3144              0.011213  maktaba#ensure#TypeMatches()
 2517              0.009126  maktaba#path#RootComponent()
  132   0.124043   0.007136  maktaba#plugin#GetOrInstall()
 2006   0.013524   0.006436  maktaba#ensure#IsList()
  147   0.009870   0.005441  maktaba#string#Strip()
  148   0.072684   0.005021  maktaba#function#Call()
   92   0.057603   0.004935  maktaba#plugin#Enter()
  171   0.005266   0.004645  maktaba#path#Split()
  610   0.023335   0.004571  maktaba#setting#Handle()
  175   0.005383   0.004011  maktaba#flags#Create()
  159   0.004876   0.003906  maktaba#rtp#Split()
   77   0.017688   0.003549  maktaba#plugin#AddonInfo()
   80   0.067771   0.003027  maktaba#plugin#Source()

Profile summary after this change:

FUNCTIONS SORTED ON SELF TIME
count  total (s)   self (s)  function
  874   0.070339   0.042532  maktaba#path#Join()
 2867              0.030666  maktaba#path#StripTrailingSlash()
   42   0.043343   0.026018  maktaba#rtp#LeafDirs()
  658   0.021416   0.014772  maktaba#setting#ParseHandle()
  610   0.057293   0.012579  maktaba#plugin#Flag()
  559   0.018096   0.012517  maktaba#value#Focus()
 3144              0.011521  maktaba#ensure#TypeMatches()
   41   0.041070   0.011328  maktaba#rtp#Join()
  808              0.010794  maktaba#path#AsDir()
 2517              0.009276  maktaba#path#RootComponent()
  132   0.125767   0.007175  maktaba#plugin#GetOrInstall()
 2006   0.013761   0.006481  maktaba#ensure#IsList()
  147   0.009712   0.005320  maktaba#string#Strip()
  148   0.080790   0.005243  maktaba#function#Call()
   92   0.058992   0.005129  maktaba#plugin#Enter()
  171   0.005646   0.005018  maktaba#path#Split()
  610   0.023532   0.004599  maktaba#setting#Handle()
  175   0.005448   0.004046  maktaba#flags#Create()
  159   0.004840   0.003883  maktaba#rtp#Split()
   77   0.017444   0.003549  maktaba#plugin#AddonInfo()

Actually seems to save a little overhead by replacing the s:StripTrailingSlash hack in rtp.vim with a simpler substitution (instead of using #path#AsDir and slicing). Certainly not a significant performance regression, at any rate.

Fixes #137 and #175.

[malcolmr]

LGTM

[malcolmr]

Shouldn't this be ['/absolute', 'path']?

[dbarnett]

Good catch. Fixed.

[dbarnett]

FYI, I found a hacky one-off s:StripTrailingSlash I could remove in favor of the real implementation, and then I did a little profiling because I was concerned about hotspot performance regressions (see updated PR description). Everything looks good there.