txscript: Optimize IsUnspendable.
txscript: Optimize IsUnspendable.

This converts the IsUnspendable function to make use of a combination of
raw script analysis and the new tokenizer instead of the far less
efficient parseScript thereby significantly optimizing the function.

It is important to note that this new implementation intentionally has a
semantic difference from the existing implementation in that it will now
report scripts that are larger than the max allowed script size are
unspendable as well.

Finally, the comment is modified 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
-----------------------------------------------------------
BenchmarkIsUnspendable   149899       860          -99.43%

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

benchmark                old bytes    new bytes    delta
-----------------------------------------------------------
BenchmarkIsUnspendable   466945       0            -100.00%

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