Rename shuffle to swizzle

This is in preparation for replacing shuffleMix with a fully-general
two-input shuffle, to be named 'shuffle'.
Replace shuffleMix with a fully-general shuffle

https://github.com/johnmccutchan/ecmascript_simd/issues/74
Tests for int32x4 shuffle and swizzle
Switch from swizzle masks to swizzle indices

This changes SIMD.float32x4.swizzle(x, SIMD.XYZW) to
SIMD.float32x4.swizzle(0, 1, 2, 3). This gives us scalability for
int8x16, and consistency with SIMD.float32x4.shuffle, which has already
made this transition.
Merge pull request #83 from sunfishcode/shuffle

New-style shuffles