Make serializer dump() non-recursive

[vii]

This allows printing very deep trees without running out of stack.

[coveralls]

coverage: 100.0%. remained the same
when pulling ccbafb4 on vii:develop
into 960b763 on nlohmann:develop.

[vii]

Unfortunately has some compile errors on MSVC etc. Can update and fix if you like this direction -

error C7555: use of designated initializers requires at least '/std:c++20'

[gregmarr]

I'm not saying whether this is something that should be pursued or not, but in case it is, a couple initial observations.

[gregmarr]

By taking a reference here, you end up with a dangling reference when you do pop_back(). Even if you don't use state again after the pop_back(), you are referring to the BasicJsonType inside it with val. It's probably better to store a BasicJsonType * in the state, so you don't have to worry about referenceds to references, and then you can do this:

const BasicJsonType* val = stack.back().val;
const unsigned int current_indent = stack.back().new_indent;
const unsigned int offset = stack.back().offset;
auto iterator = stack.back().iterator;
stack.pop_back();

[gregmarr]

A std::stack would probably be a better choice.

[t-b]

I very much like this fix here! It was on my todo list as well.