Allow empty argument list methods.

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

test/function/empty_body.wren

@@ -1,2 +1,2 @@
 var f = new Fn {}
-IO.print(f.call) // expect: null
+IO.print(f.call()) // expect: null

test/function/newline_body.wren

@@ -1,4 +1,4 @@
 var f = new Fn {
   // Hi.
 }
-IO.print(f.call) // expect: null
+IO.print(f.call()) // expect: null

test/function/newline_in_expression_block.wren

@@ -1,5 +1,5 @@
 new Fn { IO.print("ok") // expect error
-}.call // expect error
+}.call() // expect error
 
 // The second error is cascaded here. If it starts failing, just remove that
 // expectation.

test/function/no_parameters.wren

@@ -0,0 +1 @@
+new Fn {|| null } // expect error

test/function/parameters.wren

@@ -1,5 +1,5 @@
 var f0 = new Fn { 0 }
-IO.print(f0.call) // expect: 0
+IO.print(f0.call()) // expect: 0
 
 var f1 = new Fn {|a| a }
 IO.print(f1.call(1)) // expect: 1

test/function/syntax.wren

@@ -1,16 +1,16 @@
 // Single expression body.
-new Fn { IO.print("ok") }.call // expect: ok
+new Fn { IO.print("ok") }.call() // expect: ok
 
 // Curly body.
 new Fn {
   IO.print("ok") // expect: ok
-}.call
+}.call()
 
 // Multiple statements.
 new Fn {
   IO.print("1") // expect: 1
   IO.print("2") // expect: 2
-}.call
+}.call()
 
 // Extra newlines.
 new Fn {
@@ -22,4 +22,4 @@ new Fn {
   IO.print("2") // expect: 2
 
 
-}.call
+}.call()