refactor: split the library into fumi-core and make it embeddable

cli/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "fumi"
+version = "0.1.0"
+edition = "2021"
+description = "Smol Mail client"
+license = "Apache-2.0"
+
+[[bin]]
+name = "fumi"
+path = "src/main.rs"
+
+[features]
+# Passes the core's carrier feature through; addresses select the carrier by
+# scheme, so enabling it only affects smol+rns:// dials.
+rns = ["fumi-core/rns"]
+
+[dependencies]
+fumi-core = { path = "../core" }
+clap = { version = "4", features = ["derive"] }
+anyhow = "1"
+rand_core = { version = "0.6", features = ["getrandom"] }
+data-encoding = "2"