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 12858692080

Details

• 15 of 39 (38.46%) changed or added relevant lines in 5 files are covered.
• 12 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 78.439%

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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%
ergotree-ir/src/serialization/types.rs	11	78.77%

Totals
Change from base Build 12853076213:	-0.1%
Covered Lines:	11125
Relevant Lines:	14183

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💛 - Coveralls