Use the std stub definitions from the 'avr-std-stub' crate

Cargo.toml

@@ -20,6 +20,7 @@ build = "core_generator/build.rs"
 keywords = ["avr", "arduino", "uno"]
 
 [dependencies]
+avr-std-stub = "1.0"
 target-cpu-macro = "0.1"
 
 [build-dependencies]

src/lib.rs

@@ -9,6 +9,8 @@
 
 #![no_std]
 
+extern crate avr_std_stub;
+
 pub use self::register::{Register, RegisterBits, RegisterValue};
 pub use self::pin::{DataDirection, Pin};