This refactoring CL moves all instantiation logic in its own file,
separating it from the module compiler.

R=ahaas@chromium.org

Change-Id: I5a721c7357022dd7bf32f776b2ab0153f7dd68fc
Reviewed-on: https://chromium-review.googlesource.com/c/1409429
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58797}