Separated:
- decoding of locals
- loop assignment analysis
- determination of opcode length

as statics that work on a Decoder. Neither need the context of a
Module, and were used in scenarios where one wasn't available either.

Changed BodyLocalDecls to match the usecases for the type. In all but
one (a printer), we want the list (in order of declaration, with
repetitions) of types of locals.

Removed a now-unnecessary constructor for the WasmFullDecoder.

BUG=

Review-Url: https://codereview.chromium.org/2610813009
Cr-Commit-Position: refs/heads/master@{#42115}