Implement Uint::from_be_slice()

Needed because Rust 1.29 does not easily allow converting from a slice
into an array.

src/util/uint.rs

@@ -90,9 +90,15 @@ macro_rules! construct_uint {
                 }
             }
 
-            /// Creates big integer value from a byte slice array using
+            /// Creates big integer value from a byte array using
             /// big-endian encoding
             pub fn from_be_bytes(bytes: [u8; $n_words * 8]) -> $name {
+              Self::from_be_slice(&bytes)
+            }
+
+            /// Creates big integer value from a byte slice using
+            /// big-endian encoding
+            pub fn from_be_slice(bytes: &[u8]) -> $name {
                 use super::endian::slice_to_u64_be;
                 let mut slice = [0u64; $n_words];
                 slice.iter_mut()
@@ -431,7 +437,6 @@ macro_rules! construct_uint {
             fn deserialize<D: $crate::serde::Deserializer<'de>>(
                 deserializer: D,
             ) -> Result<Self, D::Error> {
-                use ::std::convert::TryInto;
                 use ::std::fmt;
                 use $crate::hashes::hex::FromHex;
                 use $crate::serde::de;
@@ -452,7 +457,7 @@ macro_rules! construct_uint {
                         if bytes.len() != $n_words * 8 {
                             Err(de::Error::invalid_length(bytes.len() * 2, &self))
                         } else {
-                            Ok($name::from_be_bytes(bytes[..].try_into().unwrap()))
+                            Ok($name::from_be_slice(&bytes[..]))
                         }
                     }
                 }