CXX-3173 Make make_unique.hpp an internal header

[eramongodb]

Summary

Resolves CXX-3173. Verified by this patch.

Removes bsoncxx/stdx/make_unique.hpp from public headers.

Note

The majority of diffs in this PR are updates to include directives and qualifiers.

make_unique

Since its introduction, this particular C++ polyfill has been unlike the other polyfills (optional and string_view). Despite make_unique.hpp being a public header under stdx/, it's usage has always and only ever been in implementation files, never in a public header (with the sole exception of a stray unused include directive in bsoncxx/types/bson_value/value.hpp). Its unique feature testing behavior has nevertheless managed to cause problems for users (#924) despite not being part of the API or ABI.

Given its dubious value as a public polyfill (+ existing efforts to avoid encouraging their use by users such as in #1182), this PR proposes moving this header into internal directories (alongside other for-internal-use-only private headers) to better reflect its purpose and use. The function templates are moved out of the bsoncxx::v_noabi::stdx namespace and into the bsoncxx::detail namespace for consistency with other internal interfaces and avoid confusion with public v_noabi (it is not part of the ABI) or stdx (it is not a configurable polyfill like optional or string_view) interfaces.

Note: mongocxx, test, and microbenchmark targets have visibility of private bsoncxx headers and so are not limited by the relocation of this header. That being said, given the microbenchmark target unconditionally requires C++17 or newer anyways, this PR updates its code to use std::make_unique instead.