Rework color editing in fish_config. Bury customization mode a bit:
initially the user only sees the various themes, and has to activate
customization mode to change colors. Tweak the appearance as well.
Optimize the tokenize_variable_array hot spot to do less string copying
Optimize some fast paths in autoload loading. Use an iterator to avoid
doing multiple set lookups, and cache the tokenized path to avoid
multiple memory allocations.