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.
People regularly ask how to make abbreviations global (i.e., private to
a fish session) rather than universal. So explain how to do so in the
`abbr` man page.
Fixes#3446
We silently upgrade existing abbreviations and change the separator when
saving.
This does not yet warn when the user is using the old syntax.
Resolves#2051
Support for space-delimited abbreviations was added to the expansion
parser in fbade198; this commit extends that support to the user-facing
tools, and documents the space-separated behaviour. Equals-delimited
abbreviations are expected to be removed before the next release.
Work on #731.
