Remove Metadata Importing Requirements for External References and Signatures - #655

This is a breaking change that aims to make generating CIL code and .NET metadata easier.

Features:

• Remove requirement for importing TypeReferences and MemberReferences.
• Split IModuleProvider::Module into ContextModule and DeclaringModule.
• IResolutionScope::CreateTypeReference automatically sets the context module for more predictable type resolution.
• Add IMemberDefinition::Resolve(ModuleDefinition) overloads taking a module definition as overriding resolution context.

This should avoid the very common problem that people run into when forgetting to import a reference and not knowing which reference was not imported exactly. It should also remove any confusion of the meaning of the Module property in metadata members that represent external references (i.e., is Module the module that owns the reference or the module the definition behind the reference is defined in?). Note that definitions (e.g., TypeDefinition) still need to be imported explicitly, to convert them to the appropriate references (i.e., TypeReference).