wasm: Add convenience aliases with unsigned names (#1174)

Naming right now for wasm simd intrinsics takes the signededness of the
instruction into account, but some operations are the same regardless of
signededness, such as `i32x4_add`. This commit adds aliases for all of
these operations under unsigned names as well (such as `u32x4_add`)
which are just a `pub use` to rename the item as two names. The goal of
this is to assist in reading code (no need to switch back and forth
between `i` and `u`) as well as writing code (no need to always remember
which operations are the same for signed/unsigned but only available
under the signed names).

Author: alexcrichton