This adds direct support for `std::string_view` when available (C++17
and above). The current API can be used with `std::string_view` via the
low-level two-pointer methods, but is not ergonomic. E.g., compare:
```
Json::Value node;
std::string foo, bar, baz;
std::string_view foo_sv, bar_sv, baz_sv;
// Efficient & readable:
node[foo][bar][baz];
// Less efficient, less readable:
node[std::string(foo_sv)][std::string(bar_sv)][std::string(baz_sv)];
// Efficient, but a lot less readable:
*node.demand(foo_sv.data(), foo_sv.data() + foo_sv.size())
->demand(bar_sv.data(), bar_sv.data() + bar_sv.size())
->demand(baz_sv.data(), baz_sv.data() + baz_sv.size())
// After this change, efficient & readable:
node[foo_sv][bar_sv][baz_sv];
```
* The constructor can take a `std::string_view` parameter. The existing
overloads taking `const std::string&` and `const char*` are still necessary
to support assignment from those types.
* `operator[]`, `get()`, `isMember()` and `removeMember()` take a
`std::string_view` parameter. This supersedes the overloads taking
`const std::string&` and `const char*`. The overloads taking a pair of
pointers (begin, end) are preserved for source compatibility.
* `getString()` has an overload with a `std::string_view` output parameter.
The one with a pair of pointers is preserved for source compatibility.
Signed-off-by: Lev Kandel <lmakhlis@google.com>
Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>
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