Issue #3678: Remove wrappers and call foreign functions directly
Add externfn macro and correctly label fixed_stack_segments
auto merge of #8535 : nikomatsakis/rust/issue-3678-wrappers-be-gone-2, r=graydon

Long-standing branch to remove foreign function wrappers altogether. Calls to C functions are done "in place" with no stack manipulation; the scheme relies entirely on the correct use of `#[fixed_stack_segment]` to guarantee adequate stack space. A linter is added to detect when `#[fixed_stack_segment]` annotations are missing. An `externfn!` macro is added to make it easier to declare foreign fns and wrappers in one go: this macro may need some refinement, though, for example it might be good to be able to declare a group of foreign fns. I leave that for future work (hopefully somebody else's work :) ).

Fixes #3678.