Don't clone env on each call of Expr::eval

[SethDusek]

Somewhat related to #737. This leads to pretty major performance improvements across the board for scripts. The most extreme example is https://ergexplorer.com/transactions#e2a90dc0f3f12d6fdc8437d72ae8cf801c25a8b1734544539eb63907629e0b4b which goes from 700-800 seconds to validate down to 2.5s. Other transactions I've tested have less extreme improvements but still helpful for both signing/validation

[coveralls]

Pull Request Test Coverage Report for Build 8915297620

Details

• 15 of 16 (93.75%) changed or added relevant lines in 8 files are covered.
• 33 unchanged lines in 17 files lost coverage.
• Overall coverage increased (+0.04%) to 80.795%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ergotree-ir/src/chain/ergo_box/register/value.rs	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
ergotree-interpreter/src/sigma_protocol/prover.rs	1	78.43%
ergo-lib/src/chain/transaction.rs	1	60.0%
ergotree-ir/src/mir/avl_tree_data.rs	1	91.3%
ergo-chain-generation/src/chain_generation.rs	1	87.33%
ergotree-interpreter/src/eval/exponentiate.rs	1	81.82%
ergotree-ir/src/serialization/sigmaboolean.rs	1	81.63%
ergo-lib/src/wallet/tx_builder.rs	1	73.83%
ergo-lib/src/wallet/derivation_path.rs	1	79.17%
ergo-rest/src/api/peer_discovery_internals/non_chrome.rs	1	74.81%
ergo-lib/src/wallet/box_selector/simple.rs	1	78.1%

Totals
Change from base Build 7307508480:	0.04%
Covered Lines:	10585
Relevant Lines:	13101

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💛 - Coveralls