Add documentation header

[aminya]

This header makes it possible to implement https://github.com/atilaneves/dpp/issues/140

The changes from the original header:

• used struct instead of typedef struct to define CXComment
• renamed unsigned to uint
• removed CINDEX_LINKAGE macro
• removed excess enum keyword from the return types as it doesn't need to be specified again when the return type is an enum

I added the wrappers based on the rust-bindgen code. I converted the iterators to D ranges.