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
---------------------------------------------------------------
BenchmarkIsPayToScriptHash   139961       0.66         -100.00%

benchmark                    old allocs   new allocs   delta
---------------------------------------------------------------
BenchmarkIsPayToScriptHash   1            0            -100.00%

benchmark                    old bytes    new bytes    delta
---------------------------------------------------------------
BenchmarkIsPayToScriptHash   466944       0            -100.00%

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