Spec callsites storing named argument names.
Prepare MVMCallsite for storing named arg names.
Start placing arg names into callsite objects.

This writes them into the bytecode file and then later reads them
back in. Not using them for anything yet; we'll be able to use them
as part of spesh, and can gradually with time migrate to using them
everywhere and then exclude them from actually being passed in call
sequences.
Update bytecode dumper to dump arg names.
Start interning named arg callsites.
Make spesh survive named arg call sites.

Doesn't do the named to positional bit yet, but this at least gets it
to not do an optimization that loses named args if they're slurped.
Collect, rather than bail on, named param ops.

Currently don't do anything much with them, but this does allow us to
spesh the other params.
Add spesh op for marking a named arg used.
Turn some named args into indexed lookups.

This avoids checking string names match up. Only done for provided
required named args where types match so far.
Handle optional named args in spesh also.
Add guards/facts for named object args.