ErgoTree constant segregation support

[greenhat]

Close #78

Todo:

• substitute ConstantPlaceholder with Constant;
• gracefully handle constant deserialization errors (produce ErgoTree);
• implement ConstantStore;
• test ConstantStore;
• add SigmaByteWrite trait;
• segregate constant on serialization;

[coveralls]

Pull Request Test Coverage Report for Build 503

• 190 of 203 (93.6%) changed or added relevant lines in 28 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+2.9%) to 70.512%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
sigma-tree/src/ast/constant.rs	1	2	50.0%
sigma-tree/src/eval/costs.rs	0	1	0.0%
sigma-tree/src/serialization/constant_placeholder.rs	19	21	90.48%
sigma-tree/src/serialization/expr.rs	14	16	87.5%
sigma-tree/src/serialization/fold.rs	0	2	0.0%
sigma-tree/src/serialization/sigma_byte_writer.rs	7	9	77.78%
sigma-tree/src/ergo_tree.rs	56	59	94.92%

Files with Coverage Reduction	New Missed Lines	%
sigma-tree/src/ergo_tree.rs	1	91.85%

Totals
Change from base Build 485:	2.9%
Covered Lines:	1198
Relevant Lines:	1699

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💛 - Coveralls