Fix duplicate vars at top level

To see the problem this patch fixes, look at the first line of
lib/cli.js:

```js
var $0, $0, ...
```

The Program node handler was attempting to deduplicate with `nub`, but
doing so over a list of JS.Identifiers, not a list of plain strings.

Since most callers of `declarationsNeededRecursive` are more interested
in the set of names than the identifier nodes, I changed it to do that.

src/compiler.coffee

@@ -134,8 +134,8 @@ generateMutatingWalker = (fn) -> (node, args...) ->
 
 declarationsNeeded = (node) ->
  return [] unless node?
- if (node.instanceof JS.AssignmentExpression) and node.operator is '=' and node.left.instanceof JS.Identifier then [node.left]
- else if (node.instanceof JS.ForInStatement) and node.left.instanceof JS.Identifier then [node.left]
+ if (node.instanceof JS.AssignmentExpression) and node.operator is '=' and node.left.instanceof JS.Identifier then [node.left.name]
+ else if (node.instanceof JS.ForInStatement) and node.left.instanceof JS.Identifier then [node.left.name]
  else []
 
 declarationsNeededRecursive = (node) ->
@@ -415,7 +415,7 @@ class exports.Compiler
  decls = nub concatMap block, declarationsNeededRecursive
  if decls.length > 0
  if options.bare
- block.unshift makeVarDeclaration decls
+ block.unshift makeVarDeclaration decls.map (name) -> new JS.Identifier(name)
  else
  # add a function wrapper
  block = [stmt new JS.UnaryExpression 'void', new JS.CallExpression (memberAccess (new JS.FunctionExpression null, [], new JS.BlockStatement block), 'call'), [new JS.ThisExpression]]
@@ -1095,7 +1095,7 @@ class exports.Compiler
 
  # TODO: comments
  generateMutatingWalker (state) ->
- state.declaredSymbols = union state.declaredSymbols, map (declarationsNeeded this), (id) -> id.name
+ state.declaredSymbols = union state.declaredSymbols, declarationsNeeded this
  {declaredSymbols, usedSymbols, nsCounters} = state
  newNode = if @instanceof JS.GenSym
  newNode = new JS.Identifier generateName this, state
@@ -1110,15 +1110,15 @@ class exports.Compiler
  usedSymbols: union usedSymbols, params
  nsCounters: nsCounters_
  newNode.body = forceBlock newNode.body
- undeclared = map (declarationsNeededRecursive @body), (id) -> id.name
+ undeclared = declarationsNeededRecursive @body
  undeclared = difference undeclared, map (variableDeclarations @body), (id) -> id.name
  alreadyDeclared = union declaredSymbols, concatMap @params, collectIdentifiers
  declNames = nub difference undeclared, alreadyDeclared
  decls = map declNames, (name) -> new JS.Identifier name
  newNode.body.body.unshift makeVarDeclaration decls if decls.length > 0
  newNode
  else generateSymbols this, state
- state.declaredSymbols = union declaredSymbols, map (declarationsNeededRecursive newNode), (id) -> id.name
+ state.declaredSymbols = union declaredSymbols, declarationsNeededRecursive newNode
  newNode
 
  defaultRule = ->