Handle walrus operator

Or rather set precedence on call_stmt and expr_stmt

Adjust pytest test_single_compile so it works now

test/simple_source/bug38/02_named_expr.py

@@ -0,0 +1,14 @@
+# From 3.8 test_named_expressions.py
+# Bug was not putting parenthesis around := below
+# RUNNABLE!
+
+"""This program is self-checking!"""
+(a := 10)
+assert a == 10
+
+# Bug was not putting all of the levels of parentheses := below
+
+(z := (y := (x := 0)))
+assert x == 0
+assert y == 0
+assert z == 0