* Remove `MatchesRegexp` from eval/vals since it is not part of the
Elvish value protocol.
* Simplify implementation of value matching in `eval/evaltest`.
* Documentation wording tweaks.
This change is a preparation step for refining all *Op types to return Exception
as the error.
Keeping Exception as a struct type will make such a change error-prone, since
a (*Exception)(nil) != error(nil), so if an *Op returns a nil *Exception, it
is not nil if the return value is stored in an error-typed variable.
As with commit #eb2a792 I acknowledge that `golint` recommendations are
controversial and should not automatically be acted on. Nonetheless,
this change fixes legitimate lint issues such as copy/paste cleanups,
style problems I introduced (`i += 1` versus `i++`) or method/var comments
that are not in the preferred form.
