xcbgen 'helpfully' transforms things to C types already so that libxcb
does not have to do so. Thus, even though the XML says that a field has
type CARD8, xcbgen will claim uint8_t. This might be a bit weird, but is
so far totally fine.

However, the type mapping that xcbgen uses is not injective. All of
CARD8, BYTE and BOOL get turned into uint8_t and the original type is
lost.

This is totally fine for libxcb, but programming languages other than C
do have built in boolean types. My personal problem is with Rust, where
providing a boolean for an integer argument causes a compiler error.
This results in (relatively) ugly "0 / 1" instead of "false / true".

This commit adds a new xml_type member to SimpleType. This type contains
the original string that appeared in the XML file.

Since libxcb creates instances of SimpleType itself and to avoid
breaking the API, the new argument to SimpleType.__init__ defaults to
None.

Signed-off-by: Uli Schlachter <psychon@znc.in>