C++ warnings for (in)equality operators

C++20 tries to compile a == b as b == a too (same for !=). It emits a warning when the 2 variants have a different typing.

The fix is simple:

• the unary version should be bool operator==(const T a) const, often the second const is missing.
• the binary version should be bool operator==(const T a, const T b)

Fixing declaration with an incorrect type should remove warnings...