Add SSTring type

[SethDusek]

Add String type, some transactions use this despite it not being very useful (only bin_ops Eq, Neq are implemented)
Closes #791

[coveralls]

Pull Request Test Coverage Report for Build 12609294595

Details

• 15 of 39 (38.46%) changed or added relevant lines in 5 files are covered.
• 13 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.03%) to 78.427%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergotree-ir/src/serialization/data.rs	7	8	87.5%
ergotree-ir/src/mir/value.rs	0	3	0.0%
ergotree-ir/src/types/stype.rs	1	4	25.0%
ergotree-ir/src/mir/constant.rs	5	12	41.67%
ergotree-ir/src/serialization/types.rs	2	12	16.67%

Files with Coverage Reduction	New Missed Lines	%
ergotree-ir/src/mir/value.rs	1	57.24%
ergo-merkle-tree/src/merkletree.rs	2	95.69%
ergo-merkle-tree/src/batchmerkleproof.rs	3	90.48%
ergotree-ir/src/serialization/types.rs	7	80.82%

Totals
Change from base Build 12589822362:	-0.03%
Covered Lines:	11095
Relevant Lines:	14147

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💛 - Coveralls