427 & 428 - AVL Tree get & getMany

[ApexTheory]

Close #427, #428

[coveralls]

Pull Request Test Coverage Report for Build 1329024200

• 160 of 168 (95.24%) changed or added relevant lines in 3 files are covered.
• 14 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 85.209%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergotree-interpreter/src/eval/savltree.rs	108	116	93.1%

Files with Coverage Reduction	New Missed Lines	%
ergotree-interpreter/src/eval/downcast.rs	6	90.59%
ergotree-ir/src/serialization/types.rs	8	84.62%

Totals
Change from base Build 1328788214:	0.2%
Covered Lines:	11481
Relevant Lines:	13474

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💛 - Coveralls

[greenhat]

Looking great! I appreciate the tests for the signatures.