Not all enums may be used as a type anywhere, which was previously the
only way to include an enum in the AST. This commit makes sure all enums
are included.
This commit renames reserved field names like `type` to `_type`, and in
turn renames those fields as well (recursively). This avoids name
clashes when a C struct contains a field named `type`, which is a
reserved keyword in Go.
For some details, see:
https://golang.org/cmd/cgo/#hdr-Go_references_to_C
