Simplify ABI story

[nikomatsakis]

I've come to believe the ABI scheme I cooked up is overly complicated. The current scheme involves a set of ABIs, some of which are "platform specific" and some of which are not, and we select the most specific one we can for a given target. I'd prefer to say that an ABI string selects exactly one ABI, and then define some meta-ABIs that may vary by platform. In addition, some ABIs may be specific to a platform, in which case invocations of functions with that ABI cannot be compiled to other platforms.

I'm thinking of a set of ABI strings like:

• C -- a meta descriptor representing "C-like conventions", as today
• fastcall -- LLVM's fastcall
• stdcall -- win32 specific
• system -- stdcall on windows, C elsewhere (borrowed from D)
• aapcs -- ARM specific
• etc

[nikomatsakis]

Nominating

[catamorphism]

1.0, backcompat