Import fmt::Write using underscore

When we use the `fmt::Write` trait it is just to call its methods, we
can therefore, without any change to the logic, use `as _` when
importing the trait. This prevents naming conflicts.

Done in preparation for importing the `io::Write` trait.

bitcoin/src/crypto/key.rs

@@ -5,7 +5,7 @@
 //! This module provides keys used in Bitcoin that can be roundtrip
 //! (de)serialized.
 
-use core::fmt::{self, Write};
+use core::fmt::{self, Write as _};
 use core::ops;
 use core::str::FromStr;
 

units/src/amount.rs

@@ -6,7 +6,7 @@
 //! We refer to the documentation on the types for more information.
 
 use core::cmp::Ordering;
-use core::fmt::{self, Write};
+use core::fmt::{self, Write as _};
 use core::str::FromStr;
 use core::{default, ops};