Re-introduce `ValueSum::from_raw` as a `pub(crate)` method

We removed this in zcash/orchard#267 as it did not need to be part of
the public API, but we do still need a way to convert the user-defined
valueBalance type into a `ValueSum` when constructing `bvk`, and this
method is preferable to exposing the `ValueSum` internals.

src/value.rs

@@ -140,6 +140,16 @@ impl ValueSum {
         // Default for i64 is zero.
         Default::default()
     }
+
+    /// Creates a value sum from its raw numeric value.
+    ///
+    /// This only enforces that the value is a signed 63-bit integer. We use it internally
+    /// in `Bundle::binding_validating_key`, where we are converting from the user-defined
+    /// `valueBalance` type that enforces any additional constraints on the value's valid
+    /// range.
+    pub(crate) fn from_raw(value: i64) -> Self {
+        ValueSum(value as i128)
+    }
 }
 
 impl Add for ValueSum {