improve fidelity of check if atom fits in SmallAtom

[arvidn]

This factors out some common code and generally simplifies and unifies the behavior. With additional tests and fuzzer checks.

The second commit addresses the use of as_unique_address(), which is used in ObjectCache as a cheap hash map (index into a Vec). However, with the SmallAtom feature, NodePtr indices is no longer dense, and it no longer works to use a Vec instead of a HashMap. It allocates too much memory if you create an atom with value 0x3ffffff.

This commit is included in this PR since the increased fidelity of matching atoms to SmallAtom pushed the fuzzer over the limit and started failing with out-of-memory.

[coveralls-official]

Pull Request Test Coverage Report for Build 7900735840

Details

• 0 of 21 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 94.301%

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Totals
Change from base Build 7888730470:	-0.04%
Covered Lines:	5742
Relevant Lines:	6089

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💛 - Coveralls

[Rigidity]

lgtm