txscript: Optimize ExtractAtomicSwapDataPushes.
txscript: Optimize ExtractAtomicSwapDataPushes.

This converts the ExtractAtomicSwapDataPushes function to make use of
the new tokenizer instead of the far less efficient parseScript thereby
significantly optimizing the function.

The new implementation is designed such that it should be fairly easy to
move the function into the atomic swap tools where it more naturally
belongs now that the tokenizer makes it possible to analyze scripts
outside of the txscript module.  Consequently, this also deprecates the
function.

The following is a before and after comparison of attempting to extract
from both a typical atomic swap script and a very large non-atomic swap
script:

benchmark                                   old ns/op    new ns/op    delta
------------------------------------------------------------------------------
BenchmarkExtractAtomicSwapDataPushes        1330         410          -69.17%
BenchmarkExtractAtomicSwapDataPushesLarge   136819       69.3         -99.95%

benchmark                                   old allocs   new allocs   delta
------------------------------------------------------------------------------
BenchmarkExtractAtomicSwapDataPushes        2            1            -50.00%
BenchmarkExtractAtomicSwapDataPushesLarge   1            0            -100.00%

benchmark                                   old bytes    new bytes    delta
------------------------------------------------------------------------------
BenchmarkExtractAtomicSwapDataPushes        3168         96           -96.97%
BenchmarkExtractAtomicSwapDataPushesLarge   466944       0            -100.00%

decred/dcrd,jrick/dcrd,jrick/dcrd,decred/dcrd