store element count as isize and clamp len to >= 0

[domenicquirl]

Changes the map's element count to be stored as a signed integer, to avoid panics if the count is updated by for example two removing and one inserting thread out of order.

In order to not report a negative size to users, len is adjusted to return 0 if the actual count is negative. This is translated from the Java implementation, but without the upper bounds check, since for us the storage data type (isize) cannot be larger than the maximal value of the return type (usize).

Note that this means that technically the same race condition still exists at the upper bound, however the map's MAXIMUM_CAPACITY is way below isize::MAX.

Closes #86.

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This change is 

[codecov]

Codecov Report

Merging #88 into master will decrease coverage by 0.00%.
The diff coverage is 85.71%.

Impacted Files	Coverage Δ
src/node.rs	77.96% <ø> (ø)
src/map.rs	84.07% <85.71%> (-0.02%)	⬇️

[domenicquirl]

@jonhoo would you mind giving these changes another test run? I ran our tests on a branch with both this and #87 applied for a good half an hour and got no hits, but I also wasn't able to reproduce the errors this is supposed to fix before, so, yeah.

[jonhoo]

Yup — I'll merge them both, and then run the test suite for a while. Thanks!