Rearrange the file tree.

It's actually, like, a tree now.  Testing is a bit wobbly: you really do
have to make the library before the tests will build.  I don't like
this, but the pieces of the library are all rather intertwined.

One small piece of unintertwining: `strongprime.c' no longer includes
`rand.h', which detaches the mathematical code from the symmetric-crypto
build system disaster.

Test files have been moved into `t/' directories as is now established
practice.

Also take the opportunity to eradicate the CVS `$Id...$' droppings.