txscript: Optimize GetSigOpCount.
txscript: Optimize GetSigOpCount.

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

A new function named countSigOpsV0 which accepts the raw script is
introduced to perform the bulk of the work so it can be reused for
precise signature operation counting as well in a later commit.  It
retains the same semantics in terms of counting the number of signature
operations either up to the first parse error or the end of the script
in the case it parses successfully as required by consensus.

Finally, this also deprecates the getSigOpCount function that requires
opcodes in favor of the new function and modifies the comment on
GetSigOpCount 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
BenchmarkGetSigOpCount-8     61051         677           -98.89%

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

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

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