Enforce type metadata immutability

[rjmholt]

Metadata types should be immutable since they describe something that has been read and cannot be changed. There are some challenges to this such as:

• Nested types may want a reference to their parent, but if the nested type constructs first it can't reference the parent, and if the parent constructs first, it can't contain the children
• Methods/properties/fields can all have the type they belong to as types or parameter signatures, or worse, nested types

Some solutions here are:

• Use a prototype pattern to build the structure, with prototypes caching their own instantiation. Then crystallise the whole metadata hierarchy in a second pass
• Use lazy instantiation. This has the benefit of possibly being more performant for workloads that are only interested in a subset of metadata, but would have overhead in general