Fixing prefix of implicit members

Implicit members are TermRefs that should have a prefix corresponding to the object of which they are a member. They used to have the ThisType of their owner before.

`implicits2` provides a way to verify that the change works. It would be good to turn this
into a more robust test at some point.  But it's not high preiority, so I prefer no test to a fragile test.
Members of stable methods

Stable methods are introduced from getters by inserting () parameters.
This change makes sure that adding () does not lose the members of an
ExprType.

Also, avoid spurious stale symbol errors in loadDenot. When something is not
a SymDenotation, no use retrying in first phase; current would fail immediately.
InfoTransformers as specific DenotTransformers

Factored out denot transformer behavior where the we transform types
in a uniform way into InfoTransformers.
ByNameTypeTree is a TypTree.

Was an oversight before.
Fix to Select(Tree, Symbol)

As a denotation need to take the symbol's info *as seen from
the qualifier type*.
New phase: Nullarify

Eliminates ExprTypes and PolyTypes over value types.
Fix Nullarify treating all 'apply' methods as Function0
Merge pull request #122 from dotty-staging/transform/nullarify

Transform/nullarify