feat: optimize pushbytes by removing unreachable minimal check

[ChrisCho-H]

In rust-bitcoin, read_scriptint checks NonMinimalPush, which makes an additional minimal check needless and PushBytes inefficient.

[apoelstra]

We should add a fuzztest which tries to decode scripts both with master and 0.12.

Can wait til after #780 gets in, since that already includes a similar fuzztest for parsing expression strings.

[apoelstra]

ACK 84d1ad5; successfully ran local tests; yep, I think you are right

[sanket1729]

Can wait til after #780 gets in, since that already includes a similar fuzztest for parsing expression strings.

Working on it this week

[apoelstra]

I'm gonna go ahead and merge this. Will add the fuzztest in a later PR.