Generalize Write impl for Vec<u8> to Vec<u8, A>

As discussed in the issue tracker for the wg-allocators working group[1], updating this implementation for allocator support was most likely just forgotten in the original PR.

[1]: rust-lang/wg-allocators#86

library/std/src/io/impls.rs

@@ -1,6 +1,7 @@
 #[cfg(test)]
 mod tests;
 
+use crate::alloc::Allocator;
 use crate::cmp;
 use crate::fmt;
 use crate::io::{
@@ -357,7 +358,7 @@ impl Write for &mut [u8] {
 /// Write is implemented for `Vec<u8>` by appending to the vector.
 /// The vector will grow as needed.
 #[stable(feature = "rust1", since = "1.0.0")]
-impl Write for Vec<u8> {
+impl<A: Allocator> Write for Vec<u8, A> {
     #[inline]
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
         self.extend_from_slice(buf);