This in particular uses free functions because it's likely to end up
more consistent with types (versus a wrapper object for InstStore).
Note, this is unlikely to have a performance impact, but if it does, we
can look into related approaches (and we've already discussed using
LTO).
Renames `PendingBlock::AddInst` to `PendingBlock::Add` because
`MakeElementAccessInst` expects the matching name to exist.
context.cpp is getting large, so I'm looking at a few ways to cut out
clusters of functions. This felt like a logical cluster of functions to
move to their own file.
Note this doesn't touch the implementation at all, beyond what's needed
to change from `Context` members to context args.
Jon Ross-Perkins
Richard Smith