Provide default annotation to validators (incl. fallback)

Without that, we may encounter weird error messages when writing
  validators without an explicit `else`. Since we automatically fill it
  with a `fail`; without annotation, it unifies to a generic parameter.

  The existing check that would look for the body being an error term is
  ill-advised as it doesn't work as soon as one adds tracing, or make
  the validator a parameterized validator. Plus, it may simply trigger
  the wrong behavior as one can now annotate a validator with _whatever_
  and get pass the type-checker by plucking a `fail` keyword as body.

crates/uplc/src/tx.rs

@@ -100,9 +100,7 @@ pub fn eval_phase_two_raw(
         .or_else(|_| MultiEraTx::decode_for_era(Era::Babbage, tx_bytes))
         .or_else(|_| MultiEraTx::decode_for_era(Era::Alonzo, tx_bytes))?;
 
-    let cost_mdls = cost_mdls_bytes
-        .map(|x| CostMdls::decode_fragment(x))
-        .transpose()?;
+    let cost_mdls = cost_mdls_bytes.map(CostMdls::decode_fragment).transpose()?;
 
     let budget = ExBudget {
         cpu: initial_budget.0 as i64,