Don't use intrinsics when they are slower.

More thorough en-/decoding tests show that sometimes the functions
that use intrinsics are slower (or not really faster) than old
plain C functions.

After this patch the encoder doesn't use these new functions
when their usefulness is questionable.

Patch-from: lvqcl <lvqcl.mail@gmail.com>
Fix typos in comments.

Patch-from: lvqcl <lvqcl.mail@gmail.com>
stream_encoder.c : ifdef cleanup.

Patch-from: lvqcl <lvqcl.mail@gmail.com>