Rewrite the pattern matching code in trans.

The old code was not well structured, difficult to understand,
and buggy.

The new implementation is completely naive, so there may be a slight
runtime performance loss. That said, adding optimizations on top of
a clear and correct implementation seems easier than trying to
fix the old mess.

Fixes issue rust-lang#19064.
Fixes issue rust-lang#26989.
Fixes issue rust-lang#26251.
Fixes issue rust-lang#18060.
Fixes issue rust-lang#24875.
Fixes issue rust-lang#23311.
Fixes issue rust-lang#20046.

Author: eefriedman

src/librustc_trans/lib.rs

@@ -29,7 +29,6 @@
 #![feature(box_syntax)]
 #![feature(const_fn)]
 #![feature(iter_cmp)]
-#![feature(iter_arith)]
 #![feature(libc)]
 #![feature(path_ext)]
 #![feature(path_ext)]