reduce unnecessary var this$ = this

With 9f96f95, all `(.0)` and `(+ 1)`-like functions became bound
functions, introducing extra `var this$ = this` declarations despite not
being needed in many cases. This commit walks that side effect back by
optimizing all bound functions to only introduce a `this$` variable if
`this` is used somewhere inside the function.

src/ast.ls

@@ -2005,7 +2005,8 @@ class exports.Fun extends Node
                 body.lines.push Return Literal \this$
             else if sscope.fun?bound
             then @bound = that
-            else sscope.assign \this$ \this
+            else if @uses-this!
+            then sscope.assign \this$ \this
         if @statement
             name                    or @carp  'nameless function declaration'
             pscope is o.block.scope or @carp 'misplaced function declaration'
@@ -2090,6 +2091,11 @@ class exports.Fun extends Node
         names.pop!
         sn(null, ...names)
 
+    uses-this: ->
+        Node::traverse-children.call this, ->
+            | it instanceof Literal and it.value is \this      => true
+            | it instanceof Fun and it.bound and it.uses-this! => true
+
 #### Class
 class exports.Class extends Node
     ({@title, @sup, @mixins, body}) -> @fun = Fun [] body

test/operator.ls

@@ -825,6 +825,10 @@ o =
     eq 3, @y
 o.f!
 
+# but don't overdo it with the `this` binding
+ok (LiveScript.compile '(.0)'  {+bare,-header} .index-of \this) < 0
+ok (LiveScript.compile '(+ 1)' {+bare,-header} .index-of \this) < 0
+
 # Unary ops as functions
 ok (not) false
 ok (!).call(null, false)