Add `extern "abi" fn` type to represent C pointers

[nikomatsakis]

Per the recent weekly meeting, we should add a type to represent C style function pointers. This type should carry an ABI. This will be the type of extern functions from C code and we will no longer generate wrappers. We will allow this type to be coerced into a Rust closure, generating the stack-switching code. Finally, top-level Rust fn items will be usable as this type, in which case we will generate the thunk to do the stack-switch in the reverse direction.

[pnkfelix]

This still needs to be implemented, but its not a blocker for 0.6

[alexcrichton]

Nominating for the feature complete milestone

[nikomatsakis]

Already implemented. I am working on a patch for the last piece, which is #3678.