implement lock_mut

tests/cfail/lock_mut.rs

@@ -0,0 +1,16 @@
+extern crate cortex_m_srp;
+
+use cortex_m_srp::{C3, C4, P2, Resource};
+
+static R1: Resource<i32, C4> = Resource::new(0);
+static R2: Resource<i32, C3> = Resource::new(0);
+
+fn j1(mut prio: P2) {
+    R1.lock_mut(
+        &mut prio, |r1: &mut i32, c3| {
+            let r2 = R2.borrow(&c3);
+            let another_r1: &i32 = R1.borrow(&c3);
+            //~^ error
+        }
+    );
+}