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 Reading multiple bytes from an input adapter? #4389 (comment) ?

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[github-actions]

🔴 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 ea8b03d on TianyiChen:msgpack-int
into 8c391e0 on nlohmann:develop.