Switch from attributes to callbacks.

[adetaylor]

This is a major change to autocxx-bindgen which aims to improve maintainability by reducing divergence from upstream bindgen. Specifically, it:

• Significantly reduces the textual diffence
• Makes the remaining changes less invasive, such that merge conflicts should be easier to resolve
• Redoes the changes such that they're more attuned to the current evolution of upstream bindgen, so it may be possible to upstream some or ideally all of these changes. (The ultimate goal is to unfork bindgen!)

See google/autocxx#124 and rust-lang#2943 for the background here.

Specifically this change:

• Removes the #[bindgen_semantic_attributes] which were added to all sorts of items. Instead,
• Much more is communicated via the existing ParseCallbacks mechansim.
• In some cases, it's still necessary to annotate individual types - in this case we generate a newtype wrapper instead of attributes.

This commit also re-enables the bindgen test suite. It does not yet add tests for all the above new functionality; that's yet to come.