Implement and test file seek, extract common "read_exactly" for reuse

[BenWiederhake]

This PR:

• Extracts the private method osmium::io::detail::PbfParser::read_exactly as a generic, easily reusable function
• Implements osmium::util::file_seek for random access in files (currently unused, see below)
• Tests the seek feature (which also uses read_exactly)

This works toward #367, which is where file_seek will be very useful.

If you think file_seek should instead live in experimental, I'd be happy to move the function there.

[BenWiederhake]

Changes since last push:

• Added #include <cassert> to read_write.hpp, since I use assert(). Found through iwyu, even though its output is incredibly noisy.

[BenWiederhake]

Changes since last push:

• Rebased on current master
• Marked read_exactly as "inline" instead of "static"
• Reproduce the same build environment to make sure there are no further such issues

[joto]

Looks good, except the two small things I wrote down.

I am getting some weird effects here that github actions seems not to be testing your newsest commit. Hopefully this sorts itself out when you add those changes.

[BenWiederhake]

Changes since last push:

• Added the cast to __int64, that seems to be the type that's also used in other MSVC-specific implementations, like resize_file.
• Removed the print from the test