Remove <algorithm> dependency from from_json.hpp

[vittorioromeo]

As discussed on Reddit, here's a PR taking a first small step towards the improvement of compilation times. Avoiding dependencies on "heavy" standard library header such as <algorithm> can help.

Here is the ClangBuildAnalyzer compilation time report:
https://gist.github.com/vittorioromeo/7a89a1e9af8cb89ee217d4e85be83270

[coveralls]

coverage: 99.187% (+0.001%) from 99.186%
when pulling 8bf6d54 on vittorioromeo:develop
into 7ddea26 on nlohmann:develop.

[nlohmann]

@vittorioromeo Thanks for the PR! Could you please sign off your commit (see https://github.com/nlohmann/json/pull/4734/checks?check_run_id=40403948884).

[gregmarr]

Rather than rewrite all of these algorithms which have been in production for many years, and potentially break the use on a user-defined type that specializes std::front_insert_iterator or std::insert_iterator, I wonder if it would be better to just create a local version of transform (and since it's just used for the side effects, not the return, it's really a copy_transform, so it could be renamed):

template<class InputIt, class OutputIt, class UnaryOp>
void copy_transform(InputIt first1, InputIt last1,
                   OutputIt d_first, UnaryOp unary_op)
{
    for (; first1 != last1; ++d_first, ++first1)
        *d_first = unary_op(*first1);
}

based on "Possible Implementation" from https://en.cppreference.com/w/cpp/algorithm/transform

[github-actions]

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