Refactor tree-walk functions into module

[gunnarx]

The function that walks the type definition tree and calls an arbitrary callback for each node is useful for several things. Initially it was used to generate the type descriptions in the IFEX specifciation and therefore placed in ifex_ast_doc.py file, but it is now already used for other purposes.

It ought to be moved from the file name indicating "doc" to a generic module. When, or after, doing that it might be possible to improve the function further, and add additional helpers that do what the current code using the walk function actually wants to achieve (e.g. get a list of all types, or something similar).