[wasm] Binary 11: br_table takes a value.
[wasm] Binary 11: Add implicit blocks to if arms.
[wasm] Binary 11: Add arities to call, return, and breaks
[wasm] Binary 11: Add experimental version.

This CL changes the encoder, decoder, and tests to use a postorder
encoding of the AST, which is more efficient in decode time and
space.

R=bradnelson@chromium.org,rossberg@chromium.org,binji@chromium.org
BUG=chromium:575167
LOG=Y

Review-Url: https://codereview.chromium.org/1830663002
Cr-Commit-Position: refs/heads/master@{#35896}