Rust: Start of cmd_list rewrite

Rust: Start of cmd_list rewrite

This is a _very_ preliminary rewrite of the cmd_list tool in rust, which
is intended to be a testing ground for a safe interface in Rust to the
core btree interface. This adds rust wrappers for:

bch_fs:		provides bch2_fs_open(), bch2_fs_stop
btree_trans:	provides bch2_trans_init(), bch2_trans_exit()
btree_iter:	provides peek, peek_and_restart, advance
bch_errcode:	implements Display (wraps bch2_err_str())
bpos:		implements Ord (wraps bpos_cmp())
bkey_s_c:	implements Display (wraps bch2_bkey_val_to_text())

and other assorted types.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>