Simplifying JSON read code

[stephenberry]

Breaking change for escaped unicode keys

Removed the option escaped_unicode_key_conversion, as it added significant complexity to the underlying code and required inefficient parsing due to heap allocations and resulted in inefficient hashing. Removing this option does not break any JSON conformance.

If you require both unescaped and escaped versions for a key, then register both of these keys with the same member pointer within glz::meta. This gives far better performance, but should still generally be avoided.

Added "glaze/json/escape_unicode.hpp", which provides glz::escape_unicode. This results in a compile time escaped version of the input string.

An example:

template <>
struct glz::meta<unicode_escaped_t>
{
   using T = unicode_escaped_t;
   static constexpr auto value = object("😀", &T::smile, escape_unicode<"😀">, &T::smile);
};

[sjanel]

This is to avoid include of <algorithm> ?

[stephenberry]

@sjanel, yes I'm slowly trying to remove large header includes where they aren't really necessary.