Prior to this PR, the compiler would call stdlib.types's
__subscript_bytes__.
However, that function performs some checks we do not need.
After this MR, folding iterates directly over the bytes
memory, saving the memory access checks and the function
calls. This gets us a speedup of about 43% less CPU time
used on Firefox.
Also, by default, the CRC32 page runs a shorter timing test.
