Make validate_witness_program_length public

The `validate_witness_program_length` function is useful to users of the
library, lets make it public.

Author: tcharding

src/primitives/decode.rs

@@ -444,7 +444,8 @@ impl<'s> CheckedHrpstring<'s> {
     /// Validates the segwit witness length rules.
     ///
     /// Must be called after the witness version byte is removed from the data part.
-    fn validate_witness_program_length(
+    #[inline]
+    pub fn validate_witness_program_length(
         &self,
         witness_version: Fe32,
     ) -> Result<(), WitnessLengthError> {