This reserved 64, which is *gigantic*.
Over all of share/**.fish, 75% of lists are empty, 99.97% are 16
elements or fewer.
Reducing this to 16 reduces memory usage for a gigantic example
script (git.fish pasted a bunch of times for a total of almost 100k
lines) by ~10% and speeds up "--no-execute" time by the same amount.
For smaller scripts it's less noticeable simply because parse time
matters less.
There are other options, like creating the vec ::with_capacity, or
using 8 instead of 16, or even letting the vec just grow
naturally (rust's vec currently grows from 0 to 4 and then doubles,
which isn't terrible for this use), but the point is that 64 is
wasteful and never comes out on top, always in the last two places
comparing a bunch of choices.
