Use avltree fork to fix empty hash

[ross-weir]

Use fork of avltree library: https://github.com/ergoplatform/ergo_avltree_rust

It contains fix for empty avltree hash not matching scala implementation

closes #731

[coveralls]

Pull Request Test Coverage Report for Build 7065447378

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 17 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.07%) to 80.754%

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Files with Coverage Reduction	New Missed Lines	%
ergo-p2p/src/peer_info.rs	1	68.42%
ergo-p2p/src/peer_spec.rs	6	81.82%
ergotree-ir/src/serialization/types.rs	10	81.88%
<!--	Total:	17		-->

Totals
Change from base Build 6506221101:	-0.07%
Covered Lines:	10599
Relevant Lines:	13125

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💛 - Coveralls