C mapping for <switch>

Autogenerating C code from XCB's xkb.xml requires mapping of the newly introduced <switch> tag. Discussion on the mailing list (special thanks to Peter Harris & Jamey Sharp) revealed additional requirements:

• <valueparam> (a special kind of X protocol list type) can in principle be rewritten to <switch> - if one would do so in practice, the relevant request interface should not change. At the same time, auxiliary functions that reside in util/aux/ at the moment could be autogenerated from the protocol description. 
• <switch> is a special list type that allows conditional inclusion of fields. In order to achieve the mentioned compatibility with request interfaces for <valueparam>, a new set of functions is needed, _serialize() and _unserialize(), that transform an easy-to-use auxiliary data structure into a character stream and vice versa, evaluating the switch conditions in each case. 
• Requests that contain <switch> statements are best called from C using a newly introduced set of _aux() functions, that take an auxiliary data type for switch as an argument and perform the necessary call to _serialize() automatically. 

The code generator is now working for a subset of <switch> statements (ie. those that consist of fixed size fields only), the remaining cases are more difficult and thus need debugging. As a prototype, I have changed the <valueparam> in the CreateWindow request (xproto.xml) to <switch>, the overhead introduced by the additional serialization process is neglible on my machine.