Rollup merge of #82862 - athre0z:generalize-vec-write-impl, r=TimDiekmann

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 impl for allocator support was most likely just forgotten previously. This PR fixes this.

r? `````@TimDiekmann`````

[1]: https://github.com/rust-lang/wg-allocators/issues/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 @@ fn flush(&mut self) -> io::Result<()> {
 /// 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);