Add error checking to `traverse-pdarisc'

If there is not satisfiable `cond' clause, the result of the `cond' expression
is `void'. These error clauses stop execution at the site of the problem and
provide a useful error message.
Add some more utilities to break apart pda-risc-enh terms
Add a `pop-assign?' predicate for assigns with a var-rhs of pop

This predicate comes up fairly often, so it should be available as a library
function.
Add `folding-search', a (sort of) fold over the pda-risc-enh ASTs

It performs a tree-traversal in an unspecified order over the AST. Since ASTs
need not be true trees, it does not add already visit nodes into the worklist.
Provide `unparse-insn*' as well.

This was a bug. That procedure should definitely be available anywhere
`unparse-insn' is available.