Add way of converting between meta types with matching structures.

[mwalcott3]

based on a suggestion by @fregate in #178

Something like:

SomeClass object{}; // Some type with a meta
auto json = glz::convert<json_t>(object);
auto object2 = glz::convert<SomeClass>(json); // We should also be able to do the reverse.

Or this:

SomeClass object{}; // Some type with a meta
json_t json{}; // Other class with compatible structure
auto json = glz::copy(object, json);

[mwalcott3]

The concern is that this adds yet another of section of code that walks the entire struct that needs to be updated when custom containers are written. So while I can easily implement this right now its likely to add technical debt since it will need to be updated when new features are added along with the other 5 areas that walk the struct.

In my opinion for this to be worth it we need to be tie the implementations of code that walks the structure together or use this code more than just converting objects.

[mwalcott3]

We have to figure out how wrappers are going to be handled before this. Right now creating a wrapper results in having to create template specializations for everything that walks the structure. This should be avoidable but the way I wanted to enable custom wrappers required template alias deduction to be completed cleanly and that is a C++20 feature that is not supported on all major compilers yet.