Fixes for rustc changes
Add much more agressive provably-false checker

Now unspendable outs are determined by attempting to create a minimal
satisfying input script. If this can't be done, the output is unspendable.
(Unfortunately this "minimal satisfying script" is not (yet) something
that can be shown to the user, since it is more a bundle of constraints
than actual data pushes.)

Current limitations:
  - OP_ADD and friends mean the checker gives the script a free pass.
    There is no fundamental reason for this, I just didn't get to it
    yet.

  - Pubkeys are checked for DER encoding but signatures aren't. This
    is because secp256k1 exposes a method for pubkeys, but not one
    for sigs :). Signatures are loosely length checked.