Export MakeScritpNum, AsSmallInt, and IsSmallInt

[martonp]

I would like to import these functions to use in https://github.com/decred/dcrdex/pull/2196. The exact same functions are exported in dcrd, but I'd rather import from here on the off chance that something changes.

Thanks!

[jcvernaleo]

The reasoning makes sense to me. Just waiting for the GH actions to run then I'll approve.

[coveralls]

Pull Request Test Coverage Report for Build 4326594580

• 16 of 19 (84.21%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.07%) to 55.294%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
txscript/standard.go	8	11	72.73%
<!--	Total:	16	19	84.21%	-->

Files with Coverage Reduction	New Missed Lines	%
mempool/mempool.go	1	66.75%
<!--	Total:	1		-->

Totals
Change from base Build 4296367184:	0.07%
Covered Lines:	26654
Relevant Lines:	48204

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💛 - Coveralls