Put vector clock serialization logic alongside other serialization logic

This way, the `clock` crate doesn't depend on the `rpc` crate.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/clock/Cargo.toml

@@ -9,4 +9,3 @@ doctest = false
 
 [dependencies]
 smallvec = { version = "1.6", features = ["union"] }
-rpc = { path = "../rpc" }