This CL (finally) makes the contract between the compiler and the module
environment clear. In order to compile a function, the caller must provide
an instance of the compiler::ModuleEnv struct, which contains references
to code, function and signature tables, memory start, etc.

R=mtrofin@chromium.org,ahaas@chromium.org

Bug: 
Change-Id: I68e44d5da2c5ad44dad402029c2e57f2d5d25b4f
Reviewed-on: https://chromium-review.googlesource.com/613880Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47418}