This is very helpful for SDL, as SDL wants you to define a function named
`SDL_main`.

...of course, it shouldn't be segfaulting, but this fixes the
build.

rs=breakage

Previously, the typechecker suppressed many but not all errors,
by suppressing errors where the actual type was either ty_err, or
a function type whose result was ty_err. Added a has_ty_err flag
to the type flags so as to suppress errors for any types involving
ty_err.

r=nmatsakis

This is an interim fix to address the "Beware!" unsoundness.  I have a
more comprehensive rewrite of mode.rs in the pipeline.

r=pcwalton

Issue 4393 BigInt tests fail on 32-bit platforms

- Make `extern fn()` assignable to any closure type, rather than
  a subtype.
- Remove unused int_ty_set and float_ty_set
- Refactor variable unification and make it more DRY
- Do fn sub/lub/glb on the level of fn_sig
- Rename infer::to_str::ToStr to infer::to_str::InferStr
- Capitalize names of various types
- Correct hashing of FnMeta
- Convert various records-of-fns into structs-of-fns.  This is both
  eliminating use of deprecated features and more forwards compatible
  with fn reform.

r=pcwalton