add build feature 'allocator-debug' to add additional checks in NodePtr

[arvidn]

The allocator-debug feature is disabled by default. When enabled, NodePtrs are validated when passed into the various Allocator functions. It detects mixing NodePtr from one Allocator with another, as well as using NodePtrs that were invalidated by a restore_checkpoint().

[arvidn]

tests are run with the new build feature here: https://github.com/Chia-Network/clvm_rs/actions/runs/15733862561/job/44341292477?pr=588

[coveralls-official]

Pull Request Test Coverage Report for Build 15733862561

Details

• 78 of 80 (97.5%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 94.364%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/test_ops.rs	10	12	83.33%

Totals
Change from base Build 15712997395:	0.04%
Covered Lines:	6982
Relevant Lines:	7399

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💛 - Coveralls