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
Discussion in issue #3295 resulted in a decisions to rename the
functions --metadata flag to --details.
This also fixes a bug in the definition of the short flags for the
`functions` command. The `-e` flag does not take an argument and
therefore should not be defined as `e:`. Notice that the long form,
`--erase`, specifies `no_argument`. This discrepency happened to work
due to a quirk of how the flag parsing loop was written.
This implements a way to use the `functions` command to perform
introspection to learn about the characteristics of a function. Such as
where it came from.
Fixes#3295
