I'm not happy with this implementation. The linear search is undesirable,
but not too bad (and maybe wouldn't even be worth caching, if this were the
whole story), but we _also_ need to prevent duplicate identifiers. We are
not going to want to perform a linear search of a linked list (effectively)
every time we add an identifier to check for uniqueness, so I think the
caching is going to have to be generalized very shortly anyway.
As it stands now, a duplicate identifier would cause an error at expansion
time. That's not what we want, but it's not terrible, because you can have
that same problem in normal circumstances without local conflicts.
But this'll be used for metavariables as well, where we absolutely _do_ want
to fail at template definition time.
DEV-13708
Mike Gerwitz
fcd25d581c