units: Re-export the AmountDecoder

As we do for other `signed` and `unsigned` types; re-export the
`AmountDecoder` from the `amount` module.

Author: tcharding

units/src/amount/mod.rs

@@ -37,6 +37,9 @@ pub use self::{
     signed::SignedAmount,
     unsigned::Amount,
 };
+#[doc(inline)]
+#[cfg(feature = "encoding")]
+pub use self::unsigned::AmountDecoder;
 #[cfg(feature = "encoding")]
 #[doc(no_inline)]
 pub use self::error::AmountDecoderError;