Boxing cleanup: erasure, post-erasure, value classes.

Introduces extractors for value class trees. Puts them to
work to make the value class tree manipulations believable.
Eliminated some boxing code in erasure which had been
marked "maybe subsumed by posterasure?" after deciding
that it had been subsumed by posterasure.

Added some same-bytecode tests involving value class boxing
(actually the lack thereof.)
Merge pull request #2145 from paulp/pr/value-class-extractors

Boxing cleanup: erasure, post-erasure, value classes.