This commit adds a few fixes neccessary for integrating the
fast C API into Blink:
- added default constructor for CFunction
- removed a bogus template specialization allowing void* params
- extended the public Isolate class

Bug: chromium:1052746
Change-Id: I4f2ba84299920e2cc9d66ec1ed59302313db6c0b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2120587
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66986}