Rename do-prove-th to find-proof, and return nil instead of signaling an error when a proof cannot be found

Rationale:

- "find-proof" is slightly more descriptive than "do-prove-th"

- This function is useful in contexts where failure to find a proof is
  not considered an error.

The scaffolding function cljtap.test.alphaleantap/test-pp has been
updated to raise an error if find-proof returns nil, preserving the
semantics of signaling an error to indicate test failure.
Simplify a trivial expression in two places: `(~'var ~x) -> fml

Two functions in cljtap.nnf match an expression, then build a new
identical copy as a return value. I think the intent is clearer, and
less error prone, if we return the original expression.