Optimization: use AnyRef map for Namer -> Typer tree handoff

And uses a map per-compilation unit, rather than one per Typer.

One small change required: we now need to clear this map in the
the interactive compiler which reuses compilation units, rather
than in the call to `Typer#reset`.
Optimation: use AnyRefMap in GenASM javaName caches

These are a hotspot in the backend.