auto merge of #5435 : nikomatsakis/rust/issue-4846-refactor-self_info, r=nikomatsakis

Refactor the self-info so that the def-id is carried in ty_self()and the fn_ctxt doesn't need any self_info field at all. Pull out explicit self transformation into `check_method`. Step towards fixing `fn(&self)` to have a distinct lifetime. (cc #4846) 

r? @catamorphism
Add AbiSet and integrate it into the AST.

I believe this patch incorporates all expected syntax changes from extern
function reform (#3678). You can now write things like:

    extern "<abi>" fn foo(s: S) -> T { ... }
    extern "<abi>" mod { ... }
    extern "<abi>" fn(S) -> T

The ABI for foreign functions is taken from this syntax (rather than from an
annotation).  We support the full ABI specification I described on the mailing
list.  The correct ABI is chosen based on the target architecture.

Calls by pointer to C functions are not yet supported, and the Rust type of
crust fns is still *u8.