Change the size of the mapping. This does not sync the contents. The size
of the file after calling this is unspecified until write is called.
May change the pointer address of memory.
pub fn setLength(mm: *MemoryMap, io: Io, new_len: usize) SetLengthError!void
pub fn setLength(mm: *MemoryMap, io: Io, new_len: usize) SetLengthError!void {
return io.vtable.fileMemoryMapSetLength(io.userdata, mm, new_len);
}