txscript: Refactor to implement tokenizer from drcd

[kcalvinalvin]

parseScriptTemplate will allocate a copy of the script onto memory,
leading to excessive memory allocation. This commit brings the tokenizer
to btcd and modifies it to fit with Bitcoin.

[coveralls]

Pull Request Test Coverage Report for Build 530188701

• 853 of 1025 (83.22%) changed or added relevant lines in 9 files are covered.
• 19 unchanged lines in 5 files lost coverage.
• Overall coverage increased (+0.03%) to 53.426%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
mempool/policy.go	0	1	0.0%
txscript/tokenizer.go	72	76	94.74%
txscript/sign.go	22	28	78.57%
txscript/script.go	199	224	88.84%
txscript/engine.go	212	248	85.48%
txscript/standard.go	207	307	67.43%

Files with Coverage Reduction	New Missed Lines	%
txscript/engine.go	1	88.12%
txscript/sign.go	1	74.44%
txscript/pkscript.go	3	80.62%
txscript/script.go	7	89.34%
txscript/standard.go	7	76.66%

Totals
Change from base Build 512434868:	0.03%
Covered Lines:	20949
Relevant Lines:	39211

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💛 - Coveralls

[onyb]

Propose to close this in favour of #1684.