txscript: Optimize IsPushOnlyScript.
txscript: Optimize IsPushOnlyScript.

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

It also deprecates the isPushOnly function that requires opcodes in
favor of the new function and modifies the comment on IsPushOnlyScript
to explicitly call out the script version semantics.

The following is a before and after comparison of analyzing a large
script:

benchmark                       old ns/op     new ns/op     delta
BenchmarkIsPushOnlyScript-8     62412         622           -99.00%

benchmark                       old allocs     new allocs     delta
BenchmarkIsPushOnlyScript-8     1              0              -100.00%

benchmark                       old bytes     new bytes     delta
BenchmarkIsPushOnlyScript-8     311299        0             -100.00%

btcsuite/btcd,davecgh/btcd,tb00/btcd,btcsuite/btcd,tb00/btcd,ltcsuite/ltcd,davecgh/btcd,ltcsuite/ltcd