Resolves #111 by implementing a dict-like class EqualityMapping that uses equality for indexing, as opposed to hashing.

The test suite would work again with NumPy-proper, except another issue now persists if you use newer versions of Hypothesis (specifically after HypothesisWorks/hypothesis#3156)—an internal check assumes floats are being returned for xp.finfo(), which is not the case with NumPy-proper and so an unrelated error occurs for any test using xps.from_dtype()... so any test using xps.arrays() 😅 I wrote a quick hack which now bypasses this internal check in __init__.py)... it's certainly not ideal, along with my original st.floats() monkey patching, so will mull it over.