We were re-definining the FunctionSig typedef in several places. This
CL moves it to value-type.h, since it's a signature over ValueType.

R=titzer@chromium.org

Bug: v8:8238
Change-Id: Id5e8a55c7e0f98d61235e32a5e6cd12e04d26947
Reviewed-on: https://chromium-review.googlesource.com/c/1278387
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56597}