Add support for BSON element 11: uint64

[slowriot]

This PR adds support for decoding and encoding BSON element 11: 64bit unsigned int.

This resolves #3894.

Changes

• Decode element 0x11 correctly as uint64_t.
• Update existing write_bson_unsigned to encode unsigned 64bit values correctly (using 0x11 instead of signed 0x12).
• Update existing tests to expect support for uint64_t.

Reference

• https://bsonspec.org/spec.html

Pull request checklist

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

coverage: 99.984% (-0.02%) from 100.0%
when pulling 7c616a2 on slowriot:bson-element-11-uint64
into 8c391e0 on nlohmann:develop.