feat: allow assignment of values from previous phase to next phase (#26)

Author: alexander-camuto

halo2_proofs/src/plonk/prover.rs

@@ -238,8 +238,10 @@ where
             A: FnOnce() -> AR,
             AR: Into<String>,
         {
-            // Ignore assignment of advice column in different phase than current one.
-            if self.current_phase != column.column_type().phase {
+            // Ignore assignment of advice column in later phase than current one.
+            // Okay: Assigning Phase 1 value to Phase 2 column
+            // Not okay: Assigning Phase 2 value to Phase 1 column
+            if self.current_phase < column.column_type().phase {
                 return Ok(());
             }