the type_use code should consider ABI-specific classifications

[nikomatsakis]

I've been digging into the type_use code a bit more. I found at least one broken cast: it considered u64 and double to be equivalent because they have the same size/repr, but this is not valid because scalars and floats go in different registers, at least when passed as parameters.

I've patched over this problem for now using but I am not confident that other cases are correct. ABI rules can be quite complex! We should certainly be considering the ABI classifications when we decide whether a monomorphic instance can be reused.

[nikomatsakis]

I left a FIXME in the code.

[catamorphism]

Bumping to 0.6

[catamorphism]

Nominating for milestone 5, production-ready (unless it's a milestone 1 thing?)

[graydon]

accepted for production-ready milestone