segwit: Rename EncodeError::Write variant to Fmt

Its cleaner if both the `lib.rs` `EncodeError` and the
`segwit::EncodeError` use variants named uniformly.

In preparation for adding `Write` and `Fmt` variants (for
`std::io::Write` and `fmt::Write` respectively) to the `EncodeError` in
`lib.rs` rename the current `EncodeError::Write(fmt::Error)` variant to
`Fmt`.

src/segwit.rs

@@ -381,7 +381,7 @@ pub enum EncodeError {
     /// Encoding HRP, witver, and program into a bech32 string exceeds limit of 90 characters.
     TooLong(EncodedLengthError),
     /// Writing to formatter failed.
-    Write(fmt::Error),
+    Fmt(fmt::Error),
 }
 
 impl fmt::Display for EncodeError {
@@ -392,7 +392,7 @@ impl fmt::Display for EncodeError {
             WitnessVersion(ref e) => write_err!(f, "witness version"; e),
             WitnessLength(ref e) => write_err!(f, "witness length"; e),
             TooLong(ref e) => write_err!(f, "encoded string too long"; e),
-            Write(ref e) => write_err!(f, "writing to formatter failed"; e),
+            Fmt(ref e) => write_err!(f, "writing to formatter failed"; e),
         }
     }
 }
@@ -406,7 +406,7 @@ impl std::error::Error for EncodeError {
             WitnessVersion(ref e) => Some(e),
             WitnessLength(ref e) => Some(e),
             TooLong(ref e) => Some(e),
-            Write(ref e) => Some(e),
+            Fmt(ref e) => Some(e),
         }
     }
 }
@@ -428,7 +428,7 @@ impl From<EncodedLengthError> for EncodeError {
 
 impl From<fmt::Error> for EncodeError {
     #[inline]
-    fn from(e: fmt::Error) -> Self { Self::Write(e) }
+    fn from(e: fmt::Error) -> Self { Self::Fmt(e) }
 }
 
 #[cfg(all(test, feature = "alloc"))]