Parsing different shape for DECL trees.

Now that the grammar parses multiple initialised declarators in
declarations the shape of the subtrees under DECL nodes has
changed. This changes the parser to account for some of the
changes.

Next up: a clone method for Decls to copy the specifiers/type info
         into each Decl that results from a parse. The FUNC parsing
         is currently disabled as it reuses DECLs in places.