Make Default const and add some const Default impls

Full list of `impl const Default` types:

- ()
- bool
- char
- Cell
- std::ascii::Char
- usize
- u8
- u16
- u32
- u64
- u128
- i8
- i16
- i32
- i64
- i128
- f16
- f32
- f64
- f128
- std::marker::PhantomData<T>
- Option<T>
- std::iter::Empty<T>
- std::ptr::Alignment
- &[T]
- &mut [T]
- &str
- &mut str
- String
- Vec<T>

Author: estebank

tests/ui/or_fun_call.fixed

@@ -406,8 +406,8 @@ fn fn_call_in_nested_expr() {
         val: String::from("123"),
     });
 
-    let _ = opt_foo.unwrap_or_else(|| Foo { val: String::default() });
-    //~^ or_fun_call
+    // ok, `String::default()` is now `const`
+    let _ = opt_foo.unwrap_or(Foo { val: String::default() });
 }
 
 mod result_map_or {

tests/ui/or_fun_call.rs

@@ -406,8 +406,8 @@ fn fn_call_in_nested_expr() {
         val: String::from("123"),
     });
 
+    // ok, `String::default()` is now `const`
     let _ = opt_foo.unwrap_or(Foo { val: String::default() });
-    //~^ or_fun_call
 }
 
 mod result_map_or {

tests/ui/or_fun_call.stderr

@@ -240,12 +240,6 @@ error: use of `unwrap_or` to construct default value
 LL |     let _ = opt.unwrap_or({ i32::default() });
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `unwrap_or_default()`
 
-error: function call inside of `unwrap_or`
-  --> tests/ui/or_fun_call.rs:409:21
-   |
-LL |     let _ = opt_foo.unwrap_or(Foo { val: String::default() });
-   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `unwrap_or_else(|| Foo { val: String::default() })`
-
 error: function call inside of `map_or`
   --> tests/ui/or_fun_call.rs:424:19
    |
@@ -264,5 +258,5 @@ error: function call inside of `get_or_insert`
 LL |     let _ = x.get_or_insert(g());
    |               ^^^^^^^^^^^^^^^^^^ help: try: `get_or_insert_with(g)`
 
-error: aborting due to 41 previous errors
+error: aborting due to 40 previous errors