Introduce RawVec::reserve_for_push.

If `Vec::push`'s capacity check fails it calls `RawVec::reserve`, which
then also does a capacity check.

This commit introduces `reserve_for_push` which skips the redundant
capacity check, for some slight compile time speed-ups.

I tried lots of minor variations on this, e.g. different inlining
attributes. This was the best one I could find.

library/alloc/src/vec/mod.rs

@@ -1726,7 +1726,7 @@ impl<T, A: Allocator> Vec<T, A> {
         // This will panic or abort if we would allocate > isize::MAX bytes
         // or if the length increment would overflow for zero-sized types.
         if self.len == self.buf.capacity() {
-            self.reserve(1);
+            self.buf.reserve_for_push(self.len);
         }
         unsafe {
             let end = self.as_mut_ptr().add(self.len);