Add LongDouble, FloatComplex, DoubleComplex and LongDoubleComplex encodings

[madsmtm]

Found while working on objc_sys (#19), there might be others we're missing so I've marked Encoding with #[non_exhaustive].

Won't merge this before I've done a proper investigation!

[madsmtm]

We should also consider implementing RefEncode for integer types (and investigate the String encoding on i8/u8) Done in #52

[madsmtm]

And maybe RefEncode (though not Encode, since Box is Drop) for Box? Moved to #52

[madsmtm]

And forwarding impls for Wrapping<T>? Done in #52

[madsmtm]

Does objc2_encode require knowledge of the compiler and runtime? E.g. it is often allowed to omit details like in, inout, oneway and so on, but does this change significantly across compilers and/or runtimes?

[madsmtm]

An long double and the three _Complex floating point types are now supported, other encodings can be added on a case-by-case basis (since Encoding is #[non_exhaustive]). See also #68