Rename the project to Echo (was fedserv)

Rebrand the daemon and workspace: the binary is now `echo`, every crate
is `echo-*` (`echo-api` and the module crates), Rust imports use
`echo_*`, the gRPC proto is `proto/echo.proto` with package `echo.v1`,
and the log filter, gossip peer-name default, and on-disk store default
(`echo.db.jsonl`) follow. Docs updated throughout. No behaviour change;
crate directories and the config schema are untouched.

modules/chanfix/src/lib.rs

@@ -6,7 +6,7 @@
 //! SCORES <#channel> shows the standings; CHANFIX <#channel> performs the fix.
 //! `lib.rs` holds the dispatcher; each command lives in its own file.
 
-use fedserv_api::{NetView, Sender, Service, ServiceCtx, Store};
+use echo_api::{NetView, Sender, Service, ServiceCtx, Store};
 
 #[path = "scores.rs"]
 mod scores;