hashes: Move FromSliceError to submodule

Add a public `error` module and move the one error for the `hashes`
crate into it.

Re-export at the crate root.

Done in preparation for adding an additional error type.

hashes/src/error.rs

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: CC0-1.0
+
+//! Error code for the `hashes` crate.
+
+use core::fmt;
+
+/// Attempted to create a hash from an invalid length slice.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct FromSliceError {
+    pub(crate) expected: usize,
+    pub(crate) got: usize,
+}
+
+impl FromSliceError {
+    /// Returns the expected slice length.
+    pub fn expected_length(&self) -> usize { self.expected }
+
+    /// Returns the invalid slice length.
+    pub fn invalid_length(&self) -> usize { self.got }
+}
+
+impl fmt::Display for FromSliceError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "invalid slice length {} (expected {})", self.got, self.expected)
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for FromSliceError {}

hashes/src/lib.rs

@@ -96,6 +96,7 @@ pub mod _export {
 mod internal_macros;
 
 pub mod cmp;
+pub mod error;
 pub mod hash160;
 pub mod hkdf;
 pub mod hmac;
@@ -127,6 +128,7 @@ use core::{convert, fmt, hash};
 #[rustfmt::skip]                // Keep public re-exports separate.
 #[doc(inline)]
 pub use self::{
+    error::FromSliceError,
     hkdf::Hkdf,
     hmac::{Hmac, HmacEngine},
 };
@@ -317,30 +319,6 @@ fn incomplete_block_len<H: HashEngine>(eng: &H) -> usize {
     (eng.n_bytes_hashed() % block_size) as usize
 }
 
-/// Attempted to create a hash from an invalid length slice.
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub struct FromSliceError {
-    expected: usize,
-    got: usize,
-}
-
-impl FromSliceError {
-    /// Returns the expected slice length.
-    pub fn expected_length(&self) -> usize { self.expected }
-
-    /// Returns the invalid slice length.
-    pub fn invalid_length(&self) -> usize { self.got }
-}
-
-impl fmt::Display for FromSliceError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "invalid slice length {} (expected {})", self.got, self.expected)
-    }
-}
-
-#[cfg(feature = "std")]
-impl std::error::Error for FromSliceError {}
-
 #[cfg(test)]
 mod tests {
     use super::*;