Optimize binary `get_number` implementation by reading multiple bytes at once

[TianyiChen]

This PR improves performance for get_number implementation by reading multiple bytes at once, which saves calling overhead especially when interacting with file I/O. It adds get_elements to input adapters and allow them to select more efficient underlying calls to read multiple bytes if available

Performance for reading msgpack from C style FILE

develop

FromMsgpack/floats             166717521 ns    163715750 ns            4 bytes_per_second=52.4267Mi/s
FromMsgpack/signed_ints        171707844 ns    167527000 ns            4 bytes_per_second=51.234Mi/s
FromMsgpack/unsigned_ints      167388344 ns    164910750 ns            4 bytes_per_second=52.0468Mi/s
FromMsgpack/small_signed_ints  102578387 ns    100799000 ns            7 bytes_per_second=46.3689Mi/s

branch:

FromMsgpack/floats              61241576 ns     60370091 ns           11 bytes_per_second=142.174Mi/s
FromMsgpack/signed_ints         67659257 ns     62698083 ns           12 bytes_per_second=136.895Mi/s
FromMsgpack/unsigned_ints       59830260 ns     57518000 ns           13 bytes_per_second=149.224Mi/s
FromMsgpack/small_signed_ints   63264757 ns     61850167 ns           12 bytes_per_second=75.5688Mi/s

Questions:

• for get_elements in wide_string_input_adapter, I encountered a compile error without it. If I don't implement it, tests are passing locally, which seems to make sense: if wchar is used, likely the content has non-ASCII text and we won't want to interpret it as binary numbers, where get_elements is currently used. I am not sure are we expected to fallback to reading each char one-by-one or doing something else. Currently it just falls back to the get_character method.

• the current benchmark doesn't contain one which reads/writes binary from file, so the benchmark won't change with the PR unless those tests are included, shall we add them like https://github.com/nlohmann/json/discussions/4389#discussioncomment-9684116 ?

• • *

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• [X] Changes are described in the pull request, or an existing issue is referenced.
• [ ] The test suite compiles and runs without error.
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• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
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[github-actions[bot]]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @TianyiChen Please read and follow the Contribution Guidelines.

[coveralls]

coverage: 99.951% (-0.05%) from 100.0% when pulling ea8b03d86452f42b8512144e6fd68020e9d7cd82 on TianyiChen:msgpack-int into 8c391e04fe4195d8be862c97f38cfe10e2a3472e on nlohmann:develop.