remove library from bcachefs-tools Rust package

remove library from bcachefs-tools Rust package

When bcachefs was a C program that had some functions implemented in
Rust, it was necessary to make a static library containing the Rust
functions available for the C program to link.

Now that bcachefs is a Rust program, that library is no longer needed.
Instead, the Rust executable links in libbachefs.a.

This patch updates the crate structure to reflect that. The command
functions are moved into their own module.

There could be a need to create a "libbachefs-tools" library in the
future that exposes an API for bcachefs functionality to other
userspace programs. That will be a different, external API as opposed to
the previous library functions which were an internal API for the
bcachefs tool itself.

Signed-off-by: Thomas Bertschinger <tahbertschinger@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>