[HoneyryderChuck]

This PR adds the following changes:

Support type aliases as module self types

Module self types can now be type aliases (`module Foo : foo`), including
generic and namespaced ones, both in RBS files and in inline
`@rbs module-self:` annotations. The alias is expanded into the class
instance and interface types it's composed of, so an alias to an
intersection is equivalent to listing each component as a self type.

    interface _Reader[T]
      def read: () -> T
    end

    module Writer
      def write: (String) -> void
    end

    type io[T] = _Reader[T] & Writer

    # Same as `module Buffered : _Reader[String], Writer`
    module Buffered : io[String]
    end

    # @rbs module-self: io[String]
    module InlineBuffered
    end

Aliases expanding to other types (e.g. unions) raise the new
`InvalidSelfTypeError`, and recursive aliases raise
`RecursiveTypeAliasError`.

Support intersections in module self types

Self types can now be separated with `&` in addition to `,`, and an
intersection is equivalent to listing each component as a self type.

    module Persistent
    end

    module Stream
    end

    # Same as `module GRPC : Persistent, Stream`
    module GRPC : Persistent & Stream
    end

    # Components can be classes, modules, interfaces, or type aliases
    module Foo : Persistent & _Each[String] & io[String]
    end