mmap visited files?

[phr-2002]

This is an idea for some unknown future Emacs.  I wonder if it's
feasible, when someone first visits a file, to simply mmap it instead
of reading it all into memory.  This would allow viewing and browsing
large files to start up a little faster than the current approach.

Of course if the buffer gets modified, it's messy to not read it all
in at that point.  So buffer objects would just have a flag saying
whether the contents were a 'real' buffer or just a mapped file.