Update zcash buildTx to use map for input values

[cpacia]

Fixes #113

[coveralls]

Pull Request Test Coverage Report for Build 553

• 14 of 16 (87.5%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 53.855%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zcash/sign.go	14	16	87.5%
<!--	Total:	14	16	87.5%	-->

Totals
Change from base Build 546:	0.02%
Covered Lines:	2843
Relevant Lines:	5279

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💛 - Coveralls