Require that NaNs be considered distinct when returning unique elements

[kgryte]

This PR

• resolves Specify NaN behaviour in unique() #249 by requiring uniqueness be determined based on value equality (==).
• requires that NaN values be considered distinct when returning the unique elements of an array. Accordingly, if an array contains multiple NaN values, the returned array will also contain multiple NaN values.
• specifies that signed zeros be treated as equal. As a consequence, reconstructing an array containing signed zeros is not guaranteed to return the exact same array. Treating signed zeros as equal is the current prevailing behavior among array libraries (NumPy, MXNet, TF, ...).

[kgryte]

The changes introduced in this PR were discussed and approved during the consortium meeting held on 21 October 2021. Will merge. Any further changes can be addressed in follow-up PRs.