Rewrite the `abbr` function to store each abbreviation in a separate
variable. This greatly improves the efficiency. For the common case
it is 5x faster. For pathological cases it is upwards of 100x faster.
Most people should be able to unconditionally define abbreviations in
their config.fish without a noticable slow down.
Fixes#4048
The abbr function doesn't have the possiblity to rename abbreviations.
You have to delete the old one and create a new one. This commit adds
this functionality and uses the syntax:
abbr -r OLD_KEY NEW_KEY
Fixes#2155.
