Fixed #264 - failure to typecheck GADTs

The previous scheme derived the right bounds, but then failed to use them
because a TypeRef already has a set info (its bounds). Changing the bounds in
the symbol by a side effect does not affect that. This is good! But it showed
that the previous scheme was too fragile because it used a sneaky side effect
when updating the symbol info which failed to propgate into the cached
info in TypeRef.

We now keep GADT computed bounds separate form the symbol info
in a map `gadt` in the current context.
Made gadt map better encapsulated.
Merge pull request #271 from dotty-staging/fix/i268-gadts

Fixed #264 - failure to typecheck GADTs