Better namespace resolution for namespaced types

Fixes #67. More specifically,

The Clang parser now reports inheritance relations using fully qualified names. This allows Bindgen to pick up inherited classes in namespaces.
Binding type names are guaranteed not to include ::, by replacing them with underscores.
C++ BgInherit structs now inherit constructors of namespaced types properly.
The names of BgJumptable and BgInherit structs now include any enclosing namespaces of the C++ types (because these structs are all emitted to the top-level C++ namespace).

A consequence of these fixes is that namespaced types can be mapped directly:

classes:
  Module::Super: Module::Super

types:
  # no longer required, Bindgen always does this now
  # Module::Super: { binding_type: Module_Super }

I have also confirmed that this doesn't break qt5.cr (it merely removes a bunch of redundant Binding::s under lib Binding).

Papierkorb / bindgen

Better namespace resolution for namespaced types #83