This CL introduces the structures for JIT-ing wasm on the native heap.
They are described in detail at go/wasm-on-native-heap-stage-1

Briefly:
- WasmCodeManager manages memory for modules and offers an interior
pointer lookup (i.e. PC -> WasmCode)
- WasmCode represents code, including reloc info. It holds wasm
specific data, like function index, and runtime information, like trap
handler info.
- NativeModule manages memory for one module.

Tests cover the allocation and lookup aspects, following that current
regression tests cover the JITed code. A separate CL will enable JITing
using the new data structures.

Bug: v8:6876
Change-Id: I1731238409001fe97c97eafb7a12fd3922da6a42
Reviewed-on: https://chromium-review.googlesource.com/767581
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49501}