Factor the 'std_stub' module out into the 'avr-std-stub' crate

This way it can be reused by other crates.

src/lib.rs

@@ -24,6 +24,4 @@ pub mod config;
 
 mod register;
 mod pin;
-#[doc(hidden)]
-pub mod std_stub;
 

src/std_stub.rs

@@ -1,15 +0,0 @@
-//! Stub methods that `libstd` normally defines.
-
-// These do not need to be in a module, but we group them here for clarity.
-pub mod std {
-    #[lang = "eh_personality"]
-    #[no_mangle]
-    pub unsafe extern "C" fn rust_eh_personality(_state: (), _exception_object: *mut (), _context: *mut ()) -> () {
-    }
-
-    #[panic_handler]
-    fn panic(_info: &::core::panic::PanicInfo) -> ! {
-        loop {}
-    }
-}
-