This resolves #153 by overhauling the way we handle values passed through the interop boundary:

• Types with immutable semantics, such as structs, records and read-only collections are serialized and copied by value
• Other non-primitive types (classes and interfaces) are considered mutable and passed by reference

Additionally, this PR:

• Removes the limitation where "by-ref" types were only detected on the static interop boundary: now there is no limit on how deep such types can appear, as long as they are "user types" (BCL types are ignored to prevent leaking the whole .NET runtime into the generated interop layer)
• Adds support for "by-ref" types inside serialized types; for example, a mutable class or interface can now appear inside a serialized struct, record or collection
• Renames the concept of "static interfaces" to "modules" and removes the limitation where assembly-level [JSExport] attribute only accepted interfaces (it now works with non-static classes as well)