EIP-12: Encode/decode box value and token amount from JSON as either a number or a string

[greenhat]

Close #345

EIP-12

• parse box value and token amount as either string or number;
• introduce to_json_dapp() in Transaction and UnsignedTransaction in Wasm that encode box value and token amount as string;
• • Transaction::to_json_dapp();
• • UnsignedTransaction::to_json_dapp();
• • encode BoxValue as string;
• • encode TokenAmount as string;
• introduce to_json_dapp() in all types that have to_json()
• change to_json() in ALL types to return text (avoid silent accuracy loss on JS object -> text on JS side);
• update changelog;

[coveralls]

Pull Request Test Coverage Report for Build 1311666546

• 31 of 31 (100.0%) changed or added relevant lines in 5 files are covered.
• 10 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.02%) to 84.965%

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Files with Coverage Reduction	New Missed Lines	%
ergotree-interpreter/src/eval/downcast.rs	3	90.59%
ergotree-ir/src/serialization/types.rs	7	84.97%

Totals
Change from base Build 1311656382:	0.02%
Covered Lines:	11184
Relevant Lines:	13163

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💛 - Coveralls