rename 'prepend' macro

[jrochkind]

It sadly conflicts with the module prepend method, which might cause problems.

Should perhaps be prepend_str, with corresponding change to append=>append_str to match.

Our backwards compatibility concerns suggest it should be deprecated and can't really be removed until a traject 4.0. Is there a way to give anyone that needs to avoid the conflict with module prepend satisfaction sooner? Perhaps, there could be a way to not mix-in those particular macros in an opt-in way.

Whether this is worth the work may depend on someone actually encountering a problem with the method name conflict.