Blender Git Loki

Use mmap() IO for reading uncompressed .blends

Instead of submitting tons of tiny IO syscalls, we can speed things up
significantly by `mmap`ing the .blend file into virtual memory and directly
accessing it.

In my local testing, this speeds up loading the Dweebs file with all its
linked files from 19sec to 10sec (on Linux).

As far as I can see, this should be supported on Linux, OSX and BSD.
For Windows, a second code path uses `CreateFileMapping` and
`MapViewOfFile` to achieve the same result.

Reviewed By: mont29, brecht

Differential Revision: https://developer.blender.org/D8246