Don't fully load Mach-O binaries in memory

[indygreg]

Currently, we fully load Mach-O binaries in memory for signing. This was convenient to implement because Rust Mach-O parsers like to use 0-copy and operate on full Mach-O data.

We could mmap the file. But copying the binary content will fault the full content and balloon memory to the size of the Mach-O. So we're back where we started in terms of resident memory usage.

Another consideration is effects on in-place signing. By buffering the entire file in memory, we don't have to worry about reads after writes like we would with updating a file in-place.

Filing this feature request to not buffer the Mach-O in memory. This is a lot of work and will require refactoring a lot of APIs. But it should result in significant performance improvements for large (1+ GB) binaries or machines with slow disks. (I'm less sympathetic to slow disks since generally people signing are using SSDs these days or any I/O slowness will get mitigated by the page cache.)

Related to #11. Please comment on that issue regarding performance concerns.