Janitor: Always use `#![no_std] for runtime lib

And call `extern crate std` when the feature is enabled.
I've read this is the good practice on how to do it.
So that the std prelude is no longer included automatically.
There is then less difference between std and and no-std build which
should avoid surprises in the CI when we use things from the prelude.

The downside is that there is a bit of churn in the tests

internal/core/sharedvector.rs

@@ -535,7 +535,7 @@ fn simple_test() {
     assert_ne!(x, y);
     let z: [i32; 3] = [1, 2, 3];
     assert_eq!(z, x.as_slice());
-    let vec: Vec<i32> = vec![1, 2, 3];
+    let vec: std::vec::Vec<i32> = std::vec![1, 2, 3];
     assert_eq!(x, vec);
     let def: SharedVector<i32> = Default::default();
     assert_eq!(def, SharedVector::<i32>::default());
@@ -561,6 +561,7 @@ fn invalid_capacity_test() {
 
 #[test]
 fn collect_from_iter_with_no_size_hint() {
+    use std::string::{String, ToString};
     struct NoSizeHintIter<'a> {
         data: &'a [&'a str],
         i: usize,
@@ -604,7 +605,7 @@ fn test_capacity_grows_only_when_needed() {
 
 #[test]
 fn test_vector_clear() {
-    let mut vec: SharedVector<String> = Default::default();
+    let mut vec: SharedVector<std::string::String> = Default::default();
     vec.clear();
     vec.push("Hello".into());
     vec.push("World".into());