Merge pull request #42 from odersky/fix/#39-checkAccessible

Fix of #39
Made TypeBoundsTrees to be TypTrees so they are eliminated by PostTyperTransform
Fix problem in TermRef.alternatives

Rewrap needs to produce alternatives with signatures. Otgerwise the new denotation will simply overwrite the old because both the overloaded TermRef and the alternative will hash to the same unique TermRef.
Fix problem comparing overloaded TermRefs

Overloaded TermRefs do not have an info, and consequently do not support =:=. Yet in Typer#checkNewOrShadowed we compared termrefs with =:=. This gives an exception if the termrefs are overloaded. The fix is to provide a new method isSameRef in TypeComparer which is called instead of =:= in Typer#checkNewOrShadowed.
Added test for overloaded access with import/inheritance conflict
Fix of accidental omission in TypeComparer
Removed useless ValDef flagged by @samuelgruetter in previous pull request