Allow non-member bitwise operators

[KendallHarter]

Currently, bitwise operators are required to be member functions (see here) and the generated code results in member functions as well. This is contrary to C++ Core Guideline C.161 which recommends using non-member functions. I've looked around but can't find any motivation for this restriction and it creates situations where unfixable asymmetry exists:

test: type = {
    operator &: (this, _ : i32) -> bool = true;
}

main: () = {
    // Compile error; no way to define in cpp2
    // [[assert: 1 & test()]]
    [[assert: test() & 1]]
}

[hsutter]

Hmm, I can't remember now why I did that. I've removed that restriction in 37cf0bf -- thanks!

For comparisons the reason I restricted them to members is because with C++20 (Cpp2-derived) symmetry there's no longer a need to write them as nonmembers. But even there I suppose there could be a motivation to write them as nonmembers in order to get comparisons on a type that doesn't provide them itself -- I'll leave that alone for now but we'll see if it's often needed.

[hsutter]

This code now works:

test: type = { }

operator &: (_:test, _:i32) -> bool = true;
operator &: (_:i32, _:test) -> bool = true;

main: () = {
    [[assert: 1 & test()]]
    [[assert: test() & 1]]
}

Thanks again.