Make c2rust just forward args to discovered subcommands

[kkysen]

This changes c2rust to be a simpler wrapper around the other c2rust-* subcommand.

Instead of c2rust having to know about all the subcommands and their arguments upfront, c2rust $subcommand $args just runs c2rust-$subcommand $args. This allows c2rust instrument to work as before (before #554), while also enabling c2rust analyze and c2rust pdg in the same way. The clap help messages are still preserved for the most part, except for the short help messages for the subcommands. Otherwise, c2rust --help works as before (while also suggesting the new subcommands), and c2rust $subcommand --help works by running c2rust-$subcommand --help (instead of clap intercepting the --help).

The way this is implemented is, first the c2rust binary's directory is searched for executables named c2rust-* to discover subcommands. This is combined with the simple list of known subcommands (["transpile", "instrument", "pdg", "analyze"]) in case they're not discovered properly and we still want to suggest them. Then we check if the first argument is one of these subcommands. If it exists, we invoke it. If it doesn't exist, but is known, we suggest building it, and it doesn't exist and isn't known (or there was no subcommand given), then we run the clap parser and let it handle arg parsing and nice error/help messages. The reason we don't have everything go through clap is that I couldn't figure out a way to have clap just forward all arguments, even ones like --metadata with hyphens (Arg::allow_hyphen_values didn't work), without requiring a leading -- argument.

[rinon]

This looks good. Can you paste an example of what a bare c2rust invocation with no args looks like now? Would save me pulling this down and building it, thanks.

[kkysen]

❯ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.04s
     Running `target/debug/c2rust`
C2Rust v0.16.0+904 (7bab0a01b 2022-08-04) dirty 2 modifications
The C2Rust Project Developers <c2rust@immunant.com>

USAGE:
    c2rust <SUBCOMMAND>

FLAGS:
    -h, --help       Prints help information
    -V, --version    Prints version information

SUBCOMMANDS:
    analyze
    help          Prints this message or the help of the given subcommand(s)
    instrument
    pdg
    transpile