Modify risc-enhanced unparser to also insert UIDs and to remove syntax

It's basically necessary to see UIDs in order to identify syntax in the printed
form. Syntax is just a mess to look at.
Fix up location of unquotes
Provide `pda-term->uid' for converting a term to a UID
Add a sane printer for pda-term's

Printing out the recursive structure is basically useless.
Fix bug in setting predecessors of insn block sub-insns

Previously, we set the predecessor of the first insn of a block to the
predecessor of the block. This creates a discrepancy because the forward chain
is longer than the backwards chain (which skips the block).

This change sets the first insn of a block to the block itself.

This bug did not affect block*s.
Provide both a forward and backward folding search
The printed form of a state should not have syntax

The printed form of syntax objects is simply confusing and unnecessary. We
cannot it back in as the same syntax object anyway.
Provide `filter-to-list' for quickly getting elements satisfying a predicate

This useful for acquiring all the pushes, pops, rejects, or accept instructions
that are present in the pda.