Overhaul `math' representation machinery.

Collect type information from the C compiler at configuration
time (using a rather complicated hack so that it works with
cross-compilers).  Read this from a Python script `mpgen' which is now
responsible for knowing all of the `mp' representation details.

Since `mpgen' generates all of the constant tables directly, we no
longer have any need for the programs `genlimits' or `mpdump' -- or the
random collection of `awk' scripts for turning `mumbletab.in' files into
`mumbletab.c' files.  And this means that we can kill `libmpbase.la'.

With this change, Catacomb is finally safe for cross-compilation.