Rework assign operations (#6)

Support "var = value", "value = var", and "var = var"
Fall back the '=' to comparsion when both operators are defined
Correctly support variable shadowing with "var := value"

Also, remove the unused Variable struct

Signed-off-by: Ming-Wei Shih <mishih@microsoft.com>

tests/interpreter/cases/default/basic.yaml

@@ -24,6 +24,14 @@ cases:
           x == 4
         }
 
+        default y = 10
+
+        z = y {
+          y
+        }
+
+        y = 20
+
         default d = "has_default"
 
         default object["key"] = "string"
@@ -51,6 +59,8 @@ cases:
       x: 5
       a: -6
       b: -6
+      y: 20
+      z: 20
       c: "has_default"
       d: "has_default"
       object: