This adds a feature which allows one to look up an export by name
without knowing what module it's in -- `lookup_global_export` on an
`InstanceContents`.
The main expected use for this is to support APIs where module A
imports a function from module B, and module B needs to access module
A's memory. B can't import it from A in the normal way, because that
would create a dependency cycle. So for now, allow B to look up A's
exported memory dynamically with `lookup_global_export`.
In the future, with reference types and possibly host bindings, we'll be
able to pass references to memory as arguments, which will obviate the
need for this mechanism.
00a4e93bcd