Remove DynType, and replace it by AnyType everywhere.

The JS operations, which were previously only allowed on
operands of type DynType, are now allowed on any operand.

Casts to DynType are obviously removed.

The rationale for this change is that casts to DynType were
always allowed, meaning that AnyType and DynType could contain
exactly the same set of values. The distinction was introduced
at some point to verify that we only used JS operations on
values explicitly asInstanceOf'ed to raw JS types, but now it
only complicated things.