Implement module imports into components

[alexcrichton]

As a step towards implementing function imports into a component this
commit implements importing modules into a component. This fills out
missing pieces of functionality such as exporting modules as well. The
previous translation code had initial support for translating imported
modules but some of the AST type information was restructured with
feedback from this implementation, namely splitting the
InstantiateModule initializer into separate upvar/import variants to
clarify that the item orderings for imports are resolved differently at
runtime.

Much of this commit is also adding infrastructure for any imports at all
into a component. For example a Linker type (analagous to
wasmtime::Linker) was added here as well. For now this type is quite
limited due to the inability to define host functions (it can only work
with instances and instances-of-modules) but it's enough to start
writing *.wast tests which exercise lots of module-related functionality.

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