Put do_* methods in SourceInfoTransformMacro group

This places all do_* methods (and two unary methods in SInt and FixedPoint
that act like do_* methods) inside the ScalaDoc group
"SourceInfoTransformMacro". Classes/objects which need information about
this group have an additional bare trait mixed in, `SourceInfoDoc`, that
provides information about the group and its priority.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>