Reimplement value() access functions

[falbrechtskirchinger]

• Reimplement the value() functions, reducing the number of overloads from 6+2 to 4+2 (the 2 deprecated json_pointer<BasicJsonType> overloads remain) and squashing a few bugs in the process.

• Update Unit tests to catch reported regressions and test more scenarios.

Fixes #3652.
Fixes #3655.

To Do:

• Clean up. (Move type traits to type_traits.hpp, etc.)
• Document changes. (Template parameter order, exposition-only signatures, ...)

[coveralls]

Coverage remained the same at 100.0% when pulling ced42d5 on falbrechtskirchinger:value-ambiguity_issue3652 into 7b6cf59 on nlohmann:develop.

[falbrechtskirchinger]

Looks good to me.

[falbrechtskirchinger]

I'm trying out a new way of writing/organizing the type traits. I want to clean up type_traits.hpp soon.

using trait_name = std::integral_constant<bool, [trait conditions]>;

This, as it turns out, is a bad idea, as it results in poor compiler diagnostics. From memory: "unmet requirement std::integral_constant<bool, false>"

The type_traits unit test should also be extended. It did safe me from a small mistake with just a singular trait being tested.

Also, I'm only including nlohmann/detail/meta/type_traits.hpp when possible, to improve compilation speed. Maybe we can optimize a few more files (class_lexer, for example).

[nlohmann]

It would be great if also someone else had a look at this. @gregmarr ?

[falbrechtskirchinger]

Fixed the typo. That's it.

[falbrechtskirchinger]

Looks good to me. (Only fixed a typo since last review.)

[nlohmann]

Looks good to me.