Better namespace resolution for namespaced types

[HertzDevil]

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).

[HertzDevil]

Qualified name is not necessary here, because copy_structure already signals the fact that the jumptable struct will be defined under lib Binding.

[HertzDevil]

Is there anything blocking this PR?