Possible overflow while mapping large files on 32 bit systems

[faulesocke]

memmap-rs/src/lib.rs

Line 129 in 9df1548

Ok(len as usize)

When mapping a file > 4GiB on a 32bit system, this line will produce an integer overflow resulting in a more-or-less "random" mapping length. The correct behaviour here would be to return an error if the requested mapping is larger than the address space.

[fogti]

possible related to #69.