Uses a WeakHashMap to constrain memory footprint of docComments
Add trait keeping comments across new Typer runs
Merge pull request #2941 from huitseeker/less-destructive-docComments-reset

Cautiously give PC more control over docComments table
Merge pull request #2957 from paulp/pr/parser-improvements

Parser and position improvements.
Convenience methods from Try[T] => {Future, Promise}[T]