Check crate exports and remove internals

[greenhat]

Close #141
Close #145

• update changelog;
• export SType;

[coveralls]

Pull Request Test Coverage Report for Build 982

• 130 of 178 (73.03%) changed or added relevant lines in 15 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 85.743%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergo-lib/src/ast/box_methods.rs	0	1	0.0%
ergo-lib/src/serialization/data.rs	20	21	95.24%
ergo-lib/src/serialization/expr.rs	0	1	0.0%
ergo-lib/src/types/stype.rs	38	39	97.44%
ergo-lib/src/ast/context_methods.rs	2	4	50.0%
ergo-lib/src/chain/transaction.rs	1	3	33.33%
ergo-lib/src/serialization/fold.rs	0	2	0.0%
ergo-lib/src/ergo_tree.rs	2	7	28.57%
ergo-lib/src/ast/expr.rs	4	11	36.36%
ergo-lib/src/ast/value.rs	47	73	64.38%

Totals
Change from base Build 978:	-0.2%
Covered Lines:	2604
Relevant Lines:	3037

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💛 - Coveralls