CollectEntryPoints.

Collect entry points for backend.
Previously this was done by cleanup.
Helper method to get all members of type.
Fix error in box\unbox logic.

TermRef's for primitive types are of primitive-type.
Fix TailRec to use Label flag.

Conflicts:
	src/dotty/tools/dotc/backend/jvm/BCodeBodyBuilder.scala
	src/dotty/tools/dotc/core/Definitions.scala
Definitions used by backend.
Fix invalid flags in lazy vals.
Better error message if position of tree isn't set.