rust-gcoap: Add Rust options for small binaries (-Os)

examples/rust-gcoap/Cargo.toml

@@ -13,6 +13,10 @@ crate-type = ["staticlib"]
 # supports no unwinding), but setting it allows builds on native without using
 # the nightly-only lang_items feature.
 panic = "abort"
+# This is a typical set of options that helps Rust binaries stay small
+lto = true
+codegen-units = 1
+opt-level = "s"
 
 [dependencies]
 riot-wrappers = { version = "^0.7.18", features = [ "set_panic_handler", "panic_handler_format", "with_coap_message", "with_coap_handler" ] }