[feature] Constructor with defaults.

[p-groarke]

Hi, I hope you are well and good amidst the current pandemic.

I've been hitting a minor snag when trying to store mx::api types in various const/global containers. It would be nice if PitchData offered a flexible value constructor to remedy this.

For example:
PitchData(Step step = Step::c, int alter = 0, double cents = 0.0, Accidental c = Accidental::none, bool = false, bool = false, bool = false, bool = false, int octave = 4);

This would enable a user to create pitches like this: PitchData p{Step::c, -1}; which is nice.

A real world use case, I am building an unordered_map to map a root to a mx::api::KeyData fifth.

namespace mx {
using namespace api;
}

namespace std {
template <>
struct hash<mx::PitchData> {
	constexpr inline size_t operator()(const mx::PitchData& p) const {
		size_t h1 = size_t(p.step) << 32;
		size_t h2 = size_t(p.alter);
		return h1 ^ h2;
	}
};
} // namespace std


namespace {
std::unordered_map<mx::PitchData, int> root_to_key_circle {
    { mx::PitchData{/* you cannot do that :( */ }, -1 },
    { mx::PitchData{/* you cannot do that :( */ }, 0 },
    { mx::PitchData{/* you cannot do that :( */ }, 1 },
// etc
};
} // namespace

Now, you may want to have different or other types of constructors, but the general idea is the same. It would be nice to be able to construct a PitchData with a few arguments.

Take care and stay safe!