Mangling: initial attempt

[Jason Merrill]

Some notes:

I thought we had decided to use a unique prefix which could not appear
in C, something like '_.'.  _[0-9] doesn't qualify; it's even in the
user's namespace.

I thought we had decided to abandon the idea of truncation (perhaps
that was decided at one of the meetings you missed).

For Compression, why the n'th most recently encoded type, rather than
the n'th encoded type?

It would be useful to remember names as well as types, in case we have
multiple instantiations of the same template in one mangling.

Jason