Feed enum field offsets to type vistors.
Export adt::trans_get_discr abstractly to the type visitor.
When repr'ing an enum value, assert that it matched some variant.
Make the enum visit callback generator more readable
Make intrinsic::Opaque more opaque
auto merge of #5778 : jld/rust/reflect-abstract-enum, r=graydon

This takes care of one of the last remnants of assumptions about enum layout.  A type visitor is now passed a function to read a value's discriminant, then accesses fields by being passed a byte offset for each one.  The latter may not be fully general, despite the constraints imposed on representations by borrowed pointers, but works for any representations currently planned and is relatively simple.

Closes #5652.