typed arrays: implement load and store swizzling

typed arrays: implement load and store swizzling

Implement load and store swizzling operations. This reduces an unneeded
back and forth between types and additionally keeps the value in the
swappable type until it is swapped. This is important for correctness
when dealing with floating point, to avoid the possibility of loading
the bits of a signaling NaN (because it isn't yet swapped) into the FPU.

This additionally produces better code (comments are mine):

gcc version 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)

setValue<double>:
  movd  %xmm0, %rax         ; fp reg -> gen reg
  bswapq  %rax              ; 64-bit byte swap
  movq  %rax, (%r15,%r12)   ; store