upgrade: Use `improvement` subclasses everywhere, not instances
This changes the source definition of optimizations to match that of formats:
a subclass with a decorator, instead of an instance passed to a function call.
Not having any instance removes the confusion between class attributes and
instance attributes, which were used interchangeably.
Differential Revision: https://phab.mercurial-scm.org/D10768
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint