add fuzzer for Allocator

[arvidn]

specifically handling of atom(), small_number() and number().

This checks certain properties that we expect to hold for any atom (conditioned on whether it's an canonical form integer or not).

This functions as a property-based test.

[coveralls-official]

Pull Request Test Coverage Report for Build 7886095739

Details

• 0 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 94.337%

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Totals
Change from base Build 7884511248:	0.002%
Covered Lines:	5780
Relevant Lines:	6127

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💛 - Coveralls

[Rigidity]

lgtm