src/lib: Add Multiaddr::protocol_stack

[John-LittleBearLabs]

A convenience for cases where which protocols is of concern but the specific addresses are not, for example https://github.com/libp2p/rust-libp2p/issues/2758

[mxinden]

Can you address the CI failures @John-LittleBearLabs?

error: single-character string constant used as pattern
Error:    --> tests/lib.rs:592:50
    |
592 |         let mut toks: Vec<&str> = addr_str.split("/").collect();
    |                                                  ^^^ help: try using a `char` instead: `'/'`
    |
    = note: `-D clippy::single-char-pattern` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_char_pattern

error: could not compile `multiaddr` due to previous error
Error: The process '/home/runner/.cargo/bin/cargo' failed with exit code 101