Add Arbitrary type for InputWeightPrediction

This type creates sane Arbitrary InputWeightPrediction types that do
not panic.  To this end, when a custom type is created by calling new()
or from_slice() constructor, only use values that would no greater than
block size 4 Mwu.

bitcoin/src/blockdata/transaction.rs

@@ -12,6 +12,8 @@
 
 use core::fmt;
 
+#[cfg(feature = "arbitrary")]
+use arbitrary::{Arbitrary, Unstructured};
 use hashes::sha256d;
 use internals::{compact_size, write_err, ToU64};
 use io::{BufRead, Write};
@@ -1141,6 +1143,38 @@ mod sealed {
     impl Sealed for super::Version {}
 }
 
+#[cfg(feature = "arbitrary")]
+impl<'a> Arbitrary<'a> for InputWeightPrediction {
+    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {
+        // limit script size to 4Mwu block size.
+        let max_block = Weight::MAX_BLOCK.to_wu() as usize;
+        let input_script_len = u.int_in_range(0..=max_block)?;
+        let remaining = max_block - input_script_len;
+
+        // create witness data if there is remaining space.
+        let mut witness_length = u.int_in_range(0..=remaining)?;
+        let mut witness_element_lengths = Vec::new();
+
+        // build vec of random witness element lengths.
+        while witness_length > 0 {
+            let elem = u.int_in_range(1..=witness_length)?;
+            witness_element_lengths.push(elem);
+            witness_length -= elem;
+        }
+
+        match u.int_in_range(0..=7)? {
+            0 => Ok(InputWeightPrediction::P2WPKH_MAX),
+            1 => Ok(InputWeightPrediction::NESTED_P2WPKH_MAX),
+            2 => Ok(InputWeightPrediction::P2PKH_COMPRESSED_MAX),
+            3 => Ok(InputWeightPrediction::P2PKH_UNCOMPRESSED_MAX),
+            4 => Ok(InputWeightPrediction::P2TR_KEY_DEFAULT_SIGHASH),
+            5 => Ok(InputWeightPrediction::P2TR_KEY_NON_DEFAULT_SIGHASH),
+            6 => Ok(InputWeightPrediction::new(input_script_len, witness_element_lengths)),
+            _ => Ok(InputWeightPrediction::from_slice(input_script_len, &witness_element_lengths)),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use hex::FromHex;