Built-in support for mmap #175

Feature Description

Specially for osfs, it would be useful to have mmap support when handling files so instead of just exposing the file descriptor, the user actually request for files to be accessed in mmap mode in a best effort basis. For example:

osfs.New("<path>", osfs.WithBestEffortMmap())

Points up for debate:

• Read/Write support.
• UX for non-supporting OS.
• API.

x-ref:

• storage: filesystem/mmap, Add PackScanner to handle large repos go-git#1776 (comment)