validate existing equality and identity semantics

[soc]

Let's avoid a design embarrassment such as rust-lang/rfcs#3535 and ensure/test that the semantics work correctly as specified in https://soc.me/languages/equality-and-identity-part3.

This means Identity compares bits, Equality offers a user-provided equality, and Hash works to facilitate patterns such as identical || equal that are commonly found in hash-based collections.

• check and test that contains uses identical || equal
• rename has to includes, check and test that it uses identical
• add missing blanket impls for Identity, because the compiler does not derive them automatically right now

note: all tests should include special floating-point values (zeros, NaNs, infinities).

[soc]

#90 has been split from this issue.