When compiling a wasm module, we initially generate placeholders for
imports, which store the index corresponding to that import. Later,
at instantiation time, we use that index to correctly link the
provided import.

In this scheme, supporting multiple instantiations requires we
preserve a template (set of unlinked compiled wasm functions) which
we clone for each instance. To avoid holding on to this template,
which may be large (wasm compiled code should be expected to be in
the order of tens of MB), we must enable cloning from an instance's
linked wasm functions.

This change is a step in that direction. Instead of assuming the wasm
functions reference placeholders, we store a table of the code objects
used for imports together with the compiled module, and use that
information to determine the index of the import. Initially, that
table contains placeholders. For instances, however, that table contains
their actual imports.

BUG=

Review-Url: https://codereview.chromium.org/2269323002
Cr-Commit-Position: refs/heads/master@{#38848}