[swift2objc] Support nested declarations

[liamappelbe]

• In the AST, replace pathComponents with a new interface, NestableDeclaration, containing child and parent links.
• In the symbolgraph, the parent/child relationship is defined by a memberOf relationship. This is the same relationship as methods and properties, so all the declaration parsing infrastructure is already there.
• Parsing a ReferredType for a nested type is easy too. They look like [Foo, ".", Bar], but Bar's token has its fully qualified globally unique ID. So we can actually just discard Foo and parse Bar normally. See _nestedTypeParselet.
• Nested declarations are code-genned inside their parent
  • This requires a bit of care when finding unique names for them. These declarations have to be named using the parent's UniqueNamer, not the global one
  • That means we have to make sure (with an assert) that we're only ever transforming the nested declaration in the scope of the parent.
• Change code-gen pipeline to pass around List<String> instead of String.
  • Previously we were doing indenting by splitting the string, adding indents, then joining. This was happening every time we indented, which is pretty inefficient.
  • Better to pass around lists of lines, and only join them at the end.
  • This caused a bunch of whitespace changes in the integration tests.

[coveralls]

coverage: 88.839% (+0.03%) from 88.809%
when pulling 0f82ecf on nested
into 4ce4816 on main.