Rearrange some fmath templates to avoid some type ambiguities
Definitions of OIIO_PURE_FUNC, OIIO_CONST_FUNC, OIIO_NOTHROW, OIIO_UNUSED
simd.h: additional functionality and fixes

abs, min, max, ceil, floor, floori, xor, shift left and right,
transpose, unary minus, specializations of shuffling for SSE3, fix buggy
int reduce_add, implement float reduce_add, fixed int4(float4)
conversion, bitcasts