Make ModuleScope internal

[stakx]

Following up from our discussion in #389 (comment), I'm proposing to make ModuleScope internal. This affects mainly SaveAssembly and LoadAssemblyIntoCache, as everything else is already internal. Those methods need new public entrypoints.

Loading a previously generated assembly into the type cache

This would now be done via IProxyBuilderWithCache.LoadAssemblyIntoCache(Assembly), which is implemented in DefaultProxyBuilder.

Introducing a new interface IProxyBuilderWithCache feels like a clean solution as long as IProxyBuilder is a public abstraction... there could be user implementations out there (unlikely, I know) that don't have a type cache. But it might also be overkill. Perhaps it's OK if such implementations simply throw NotSupportedException for the new load method; then the method could sit directly in IProxyBuilder.

Saving a generated assembly

This can already be done through PersistentProxyBuilder (on platforms supporting AssemblyBuilder.Save).

However, there would now be no way to specify the desired target file path, nor the assembly's internal name. We might want to expose those capabilities through different overloads, e.g.

-string SaveAssembly();
+void SaveAssembly(string path = ModuleScope.DEFAULT_FILE_NAME);
+void SaveAssembly(string name, string path = ModuleScope.DEFAULT_FILE_NAME);

Closes #389. There's still a few bits we could remove from the public API (in some cases we already have separate issues), but by and large, we're done.

P.S.: This assumes IProxyBuilder stays public. It if shouldn't, everything here would have to move one level up (into IProxyGenerator, where it admittedly doesn't fit quite as nicely from a conceptual POV).

[stakx]

Due to the ordering of this #if, targets that don't support full binary serialization still get this interface (but it will be empty). I chose this—rather than erasing the interface as a whole—because it is true on all targets to say that "DefaultProxyBuilder is a builder with a cache", even when you cannot batch-populate it. It also gives us more API uniformity, as DefaultProxyBuilder will implement the same interfaces for all targets.