feature: memory mapped bundled files

[capr]

The current API for loading bundled files is this:

bundle.load(filename) -> string

We want to avoid creating a possibly huge string:

bundle.load(filename) -> pointer, size

For a bundled file that's trivial, but we want the same API to support loading the same file from the disk as well, transparently, so we can support transparent upgrades. For this we need to add portable support for file mmap-ing.

another cool feature would be monkey-patching the lfs API so that it transparently works with mmapped files.

and yet another feature would be accessing the file descriptor of the mmapped file so we can feed it directly to APIs that work on fds (and maybe FILE* streams too?).

Update: we already have this in fs, just need to integrate it.