Bullet-proofing companion objects

Companion class/module computations now also work for local classes and modules. For this to work,
either one of two conditions must be met: (1) some enclosing context refers to a scope that
contains the companions. (2) the context's compilation unit has a typed tree that contains the
companions. (1) is usually true when type-checking, (2) when transforming trees.

Local companions are searched as follows: If (2) holds, we locate the statement sequence containing
the companions by searching down from the root stored in the compilation unit. Otherwise, we search
outwards in the enclosing contexts for a scope containing the
companions.