Fix compilation of input_adapter(container) in edge cases

This fixes a compilation issue with the library if trying to use containers that
don't have non-member `begin()` and `end()` functions via ADL.

This patch extends the `using std::begin` and `using std::end` declarations to
also cover the return type deduction of the input_adapter() template
specialization for containers. The previous implementation only enabled the
detection of `std::begin()` and `std::end()` in the function body, making the
specialization unusable for container types that only have member `begin()` and
`end()` functions.

It is not typical to have `using` declarations in the namespace scope in a
header file. But a C++11 implementation can't rely on fully automatic return
type deduction, and needs to rely on ADL enabled helper templates. To prevent
the using declarations leaking, they are enclosed in another nested namespace.

Travis doesn't run any tests in C++17 mode

Doctest is updated to v2.4.3

Fix warnings

Do not unconditionally redefine C++14 constructs

Clarify license of is_complete_type implementation

Fix a shadowing warning

Add MAIN_PROJECT check for test and install options

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

Doctest is updated to v2.4.1

📝  add CPM.Cmake example

Change argument name "subtype" in byte_container_with_subtype

Fix move constructor of json_ref