Mark "lightweight" functions/methods in API headers

[Olly Betts]

In the Python bindings, we need to release the GIL (Global Interpreter Lock) while executing C++ functions/methods which can do significant work. We currently do this for all methods, which adds an unnecessary overhead.

In Liam's node.js wrappers, he wants to know which functions shouldn't block.

This would probably be useful for some other bindings too.

Both of these cases would be assisted by adding macro markers to the API headers for functions/methods which are "lightweight". For the Python bindings we could extract these to automatically generate a SWIG interface file to tell SWIG not to drop the GIL for these. And we could produce a list of the methods for human inspection for the node.js case.

Obvious candidates are the version functions and get_description() methods.

[Olly Betts]

There are only markers to use when generating the bindings, so no ABI impact, hence not a 1.4.0 blocker.