Reformat metadata for exported macros

Reformat metadata for exported macros

Instead of copy-pasting the whole macro_rules! item from the original .rs file,
we serialize a separate name, attributes list, and body, the latter as
pretty-printed TTs.  The compilation of macro_rules! macros is decoupled
somewhat from the expansion of macros in item position.

This filters out comments, and facilitates selective imports.