Emit version blocks for debugging
Allow AST merging to handle structs and their fields separately

so that structs will be accepted as common, if they have the same
identifier etc., even if their fields are different. And then the fields
can be diff'ed inside the struct.
Add calling convention attribute parsing, fix procptr parsing
Fix memory leak in hAstLCS()
Show "merged in" reports only once, not for each version-specific pass
-lpng: Parse once in Linux mode, once in Win32 mode
Make AST merging much faster by using array for O(1) node lookup