Inline JSON decoding fast paths assuming most things are UTF8

Summary: A couple of decoding routines have fast paths in the case that the
character is < 0x80; this restructures the code a bit so that fast path can be
inlined, and tail-calls the more complex decoding.

Reviewed By: @jdelong

Differential Revision: D1681669

Signature: t1:1681669:1416158136:a3dc22d3ed5234566c2d9a0b394c0a71d2291c4d