Auto merge of #23832 - petrochenkov:usize, r=aturon

These constants are small and can fit even in `u8`, but semantically they have type `usize` because they denote sizes and are almost always used in `usize` context. The change of their type to `u32` during the integer audit led only to the large amount of `as usize` noise (see the second commit, which removes this noise).

This is a minor [breaking-change] to an unstable interface.

r? @aturon

src/test/run-pass/issue-13763.rs

@@ -14,7 +14,7 @@
 
 use std::u8;
 
-const NUM: usize = u8::BITS as usize;
+const NUM: usize = u8::BITS;
 
 struct MyStruct { nums: [usize; 8] }
 

src/test/run-pass/vector-sort-panic-safe.rs

@@ -57,7 +57,7 @@ impl Drop for DropCounter {
 }
 
 pub fn main() {
-    assert!(MAX_LEN <= std::usize::BITS as usize);
+    assert!(MAX_LEN <= std::usize::BITS);
     // len can't go above 64.
     for len in 2..MAX_LEN {
         for _ in 0..REPEATS {