scnlib fixes/changes/updates needed to be integrated in VTK library

[spyridon97]

This PR includes all the necessary changes to integrate scnlib in VTK/ParaView as a third-party library and be used extensively for its reading/scanning capabilities.

The summary of the changes is:

1. Export functions implemented in impl.cpp
2. Use SCN_NODISCARD everywhere
3. fast_float: support version 7+
4. gcc: fix assert usage in constexpr function
5. gcc: fix template deduction warning
6. basic_scan_file_buffer: Remove current_view size 1 assumption
7. arg_reader: Fix errors when reading last part of contiguous segments from file
8. win32/default: Enforce only 1 successfully putback guarantee

[spyridon97]

@eliaskosunen we would appreciate if you could review this PR.

[spyridon97]

@eliaskosunen friendly ping

[spyridon97]

@eliaskosunen another friendly ping.

[spyridon97]

@eliaskosunen another friendly ping.

[spyridon97]

@eliaskosunen another friendly ping.