Changes the binary encoding of multi-return blocks to contain a function type index instead of a vector of value types.

Cf. https://github.com/WebAssembly/multi-value/blob/master/proposals/multi-value/Overview.md#binary-format

Bug: v8:6672
Change-Id: I506d9323bfd6dba1e7a24c8590bcf5a08b68c433
Reviewed-on: https://chromium-review.googlesource.com/599807Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Rossberg <rossberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48453}