Typed gerbil improvements

[vyzo]

Fixes 3 issues/bugs in typed gerbil:

• references in class slot types would not expand; see the regression test
• types for mutable bindings are now inferred, as the greatest commong type of the original value and the mutation; this allows us to remove some casts -- see the unit test and the changes in loader.
• (most important) specializer generation was broken by the introduction of the implicit using for methods; this is now fixed and has a regression test so that it doesn't happen again in the future.

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