Add ability to get a Span from a PIdx, improve errors

[ratmice]

This tries to improve an error (we discussed it at the time in) #456
Tested manually with the following example changes/output.

diff --git a/lrpar/examples/calc_actions/src/calc.y b/lrpar/examples/calc_actions/src/calc.y
index 8388ba2c..7164ad55 100644
--- a/lrpar/examples/calc_actions/src/calc.y
+++ b/lrpar/examples/calc_actions/src/calc.y
@@ -10,7 +10,7 @@ Expr -> Result<u64, Box<dyn Error>>:
         $1?.checked_add($3?)
               .ok_or_else(|| Box::<dyn Error>::from("Overflow detected."))
       }
-    | Term { $1 }
+    | Term
     ;

 Term -> Result<u64, Box<dyn Error>>:

  --- stderr

  thread 'main' (2683939) panicked at lrpar/examples/calc_actions/build.rs:17:10:
  called `Result::unwrap()` on an `Err` value:
  Error at ~/grmtools/lrpar/examples/calc_actions/src/calc.y:13:7
  13|     | Term
            ^^^^ Production is missing action code

[ltratt]

Excellent!