asm-llvm: add two more test cases from bug 878505.
asm-llvm: Unsigned should not be a subtype of extern.

This is https://github.com/dherman/asm.js/issues/56
asm-llvm: minor documentation updates.
asm-llvm: Allow (almost) arbitrary parenthesization of expressions.

Still not supported: the ~~ operator and arbitrary parenthesization of
call sites (ie, plus coercion).