This CL avoids the need to reference the function tables (and signatures)
as either fixed arrays or vectors, preferring vectors.

The only place we need fixed arrays is on the compiled module, to support
serialization. When we move off the GC heap, we'll also move away
from fixed arrays in that last case.

The CL aids with getting wasm of the GC heap, by reducing the places 
and representations we'll need to change  when changing the way we 
reference fixed tables.

Bug: 
Change-Id: Id4e43905a3df39062bf2839fa72dd5d9a0fe87da
Reviewed-on: https://chromium-review.googlesource.com/588334
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46917}