add `pretty_print` method to ErgoTree and Expr

[greenhat]

Close #727

[coveralls]

Pull Request Test Coverage Report for Build 6497395486

• 7 of 12 (58.33%) changed or added relevant lines in 3 files are covered.
• 19 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.06%) to 80.743%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergotree-interpreter/src/eval.rs	2	4	50.0%
ergotree-ir/src/ergo_tree.rs	0	3	0.0%
<!--	Total:	7	12	58.33%	-->

Files with Coverage Reduction	New Missed Lines	%
ergo-p2p/src/peer_info.rs	1	68.42%
ergotree-interpreter/src/eval.rs	1	80.74%
ergo-merkle-tree/src/merkletree.rs	2	97.39%
ergo-merkle-tree/src/batchmerkleproof.rs	3	90.48%
ergo-p2p/src/peer_spec.rs	6	81.82%
ergotree-ir/src/serialization/types.rs	6	83.28%
<!--	Total:	19		-->

Totals
Change from base Build 6481852424:	-0.06%
Covered Lines:	10537
Relevant Lines:	13050

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💛 - Coveralls