[iostreams] mapped_file::size() returns the wrong size on 32-bit windows platforms for files larger than 4gb

[James Whitworth <fun4jimmy@…>]

The signature is size_t mapped_file::size() which is uint32_t for 32-bit builds. This means files larger than 4gb will return an incorrect value.

Looking at the windows implementation source for mapped_file ​https://github.com/boostorg/iostreams/blob/master/src/mapped_file.cpp around line 220. The size is queried and stored as a local boost::intmax_t but when assigning to size_ it is static_cast to the smaller type.

I think the fix is to change the size member to be always 64-bit but I'm not familiar enough with the rest of the library to know if this would have any knock on effects. I suspect this is also an issue on linux as well but I am unable to easily test that at the moment,