Fix assignment priority

The assignment operator is lower priority than any other binary
operator.

Otherwise the following:

```tiger
a := b | c
```

Would be parsed as:

```tiger
(a := b) | c
```

Instead of the expected:

```tiger
a := (b | c)
```

test/corpus/regressions.txt

@@ -0,0 +1,16 @@
+================================================================================
+Assignment precedence
+================================================================================
+
+a := b | c
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (assignment_expression
+    (identifier)
+    (operator)
+    (binary_expression
+      (identifier)
+      (operator)
+      (identifier))))