Allow empty argument list methods.

- Compile them as calls and definitions.
- Use them for call(), clear(), run(), try(), and yield().
- Update the docs.

test/map/clear.wren

@@ -1,7 +1,7 @@
 var a = {1: 1, 2: 2, 3: 3}
-a.clear
+a.clear()
 IO.print(a)       // expect: {}
 IO.print(a.count) // expect: 0
 
 // Returns null.
-IO.print({1: 2}.clear) // expect: null
+IO.print({1: 2}.clear()) // expect: null