Convert NodePtr to a newtype rather than alias

[Rigidity]

This encapsulates NodePtr as a separate type (with all necessary traits derived) rather than a type alias to i32. This prevents typos or mistakes where you would use NodePtr somewhere that expects a normal i32 or vice versa. It also allows separate implementations of traits, for example encoding and decoding a NodePtr with a CLVM allocator would be a no-op, whereas i32 is encoded as an atom.

[coveralls-official]

Pull Request Test Coverage Report for Build 5814062287

• 30 of 30 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.002%) to 93.884%

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Totals
Change from base Build 5513827559:	-0.002%
Covered Lines:	5219
Relevant Lines:	5559

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💛 - Coveralls