Initial implementation of redundant_slicing lint

tests/ui/redundant_slicing.rs

@@ -0,0 +1,11 @@
+#![allow(unused)]
+#![warn(clippy::redundant_slicing)]
+
+fn main() {
+    let x: &[u32] = &[0];
+    let err = &x[..];
+
+    let v = vec![0];
+    let ok = &v[..];
+    let err = &(&v[..])[..];
+}

tests/ui/redundant_slicing.stderr

@@ -0,0 +1,16 @@
+error: redundant slicing of the whole range
+  --> $DIR/redundant_slicing.rs:6:15
+   |
+LL |     let err = &x[..];
+   |               ^^^^^^ help: use the original slice instead: `x`
+   |
+   = note: `-D clippy::redundant-slicing` implied by `-D warnings`
+
+error: redundant slicing of the whole range
+  --> $DIR/redundant_slicing.rs:10:15
+   |
+LL |     let err = &(&v[..])[..];
+   |               ^^^^^^^^^^^^^ help: use the original slice instead: `(&v[..])`
+
+error: aborting due to 2 previous errors
+