Add tokens support in TxBuilder and ErgoBoxCandidateBuilder

[greenhat]

Close #88

Todo:

• tx builder tests for minting and burning a token;
• implement token selection in SimpleBoxSelector;
• box builder tests for adding, minting token;
• newtype for token amount;
• token tests (including minting and burning) in JS;

[coveralls]

Pull Request Test Coverage Report for Build 822

• 432 of 466 (92.7%) changed or added relevant lines in 18 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+2.2%) to 86.816%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergo-lib/src/wallet/tx_builder.rs	39	40	97.5%
ergo-lib/src/wallet/box_selector/simple.rs	215	220	97.73%
ergo-lib/src/chain/ergo_box/box_value.rs	37	49	75.51%
ergo-lib/src/chain/token.rs	22	38	57.89%

Files with Coverage Reduction	New Missed Lines	%
ergo-lib/src/wallet/tx_builder.rs	1	92.92%

Totals
Change from base Build 802:	2.2%
Covered Lines:	2318
Relevant Lines:	2670

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💛 - Coveralls