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.
Previously, `--erase` would not accept any options and wouldn't read
"--" as option-separator. Now it does like every other "command", and it
could conceivably gain e.g. a "--prefix" option.
This fails on e.g. an abbr that uses `env a=b`, like the included test demonstrates.
Unfortunately it decreases the speed again (2s vs 2.2s vs 4s original),
but correctness is more important.
