extend parse callbacks to expose discovered composite types and aliases

[mxyns]

this is my attempt at #2657

example use case: this allowed me to build a library that discovers the bindings (for composite type) generated by bindgen (https://github.com/mxyns/bindgen-bridge)

i tried to expose as little hidden internal structs as possible (like passing ItemId as a usize) but I did not feel like duplicating the definition of CompType was a good idea so I just made it public