txscript: Reduce checkmultisig allocs.
txscript: Reduce checkmultisig allocs.

Profiling allocations during an initial chain sync shows that a large
portion of the allocations due to the CHECKMULTISIG opcode are the
result of generating the list of public keys and signatures.

This eliminates those allocations as follows:

- Use slices into fixed-size stack-based backing arrays for the typical
  number of public keys and signatures
- Make the slice of signature info contiguous instead of pointers which
  not only eliminates the associated allocations, but also provides
  better cache locality

decred/dcrd,decred/dcrd