Rework expr_landor

so that it also is called from the precedence parser.  This
is complicated by the fact that something needs to be done before
the second operand is parsed in a single pass compiler, so it
doesn't quite fit into expr_infix itself.  It turns out the smallest
code changes result when expr_landor remains separate.  But it can
be tidied a bit.

tests/tests2/11_precedence.c

@@ -1,4 +1,5 @@
-#include <stdio.h>
+//#include <stdio.h>
+extern int printf(const char *, ...);
 
 int main()
 {