Allow macros to return estree objects

This change allows macros to return arbitrary [estree][1] objects as an
alternative to returning arrays and objects as usual.

This means you can now write macros that support estree features that
eslisp doesn't yet (e.g. ES6 arrow functions), as long as [escodegen][2]
supports them.  You can even use crazy experimental estree extensions
(e.g.  Facebook's [JSX][3], or anko#11; ES7 async/await), as long as your
macro can do the appropriate transformation to the core eslisp standard
that escodegen understands before returning.

[1]: https://github.com/estree/estree
[2]: https://github.com/estools/escodegen
[3]: https://github.com/facebook/jsx

src/import-macro.ls

@@ -52,7 +52,8 @@ to-compiler-form = (ast) ->
     # Objects are expected to represent atoms
     | \Object =>
       if ast.atom then atom ("" + ast.atom), macro-location-string
-      else throw Error "Macro returned object without `atom` property, or atom property set to empty string (got #{JSON.stringify ast})"
+      else
+        ast # assume the returned object is in estree form
 
     # Strings become strings as you'd expect
     | \String => string ast, macro-location-string
@@ -82,6 +83,7 @@ macro-env = (env) ->
   # Create the functions to be exposed for use in a macro's body based on the
   # given compilation environment
 
+  compile : -> it |> to-compiler-form |> env.compile
   evaluate : ->
     it |> to-compiler-form |> env.compile |> env.compile-to-js |> eval
   multi : (...args) -> multiple-statements args

test.ls

@@ -1075,6 +1075,48 @@ test "macro return intermediates may be invalid if fixed by later macro" ->
     '''
       ..`@equals` "x;"
 
+test "macro can return estree object" ->
+  esl '''
+    (macro identifier (lambda ()
+      (return (object "type" "Identifier"
+                      "name" "x"))))
+    (identifier)
+    '''
+      ..`@equals` "x;"
+
+test "macro can multi-return estree objects" ->
+  esl '''
+    (macro identifiers (lambda ()
+      (return ((. this multi)
+               (object "type" "Identifier"
+                       "name" "x")
+               (object "type" "Identifier"
+                       "name" "y")))))
+    (identifiers)
+    '''
+      ..`@equals` "x;\ny;"
+
+test "macro can multi-return estree objects as well as arrays" ->
+  esl '''
+    (macro identifiers (lambda ()
+      (return ((. this multi)
+               (object "type" "Identifier"
+                       "name" "x")
+               'x))))
+    (identifiers)
+    '''
+      ..`@equals` "x;\nx;"
+
+test "macro can compile and return parameter as estree" ->
+  esl '''
+    (macro that (lambda (x)
+      (return ((. this compile) x))))
+    (that 3)
+    (that "hi")
+    (that (c))
+    '''
+      ..`@equals` "3;\n'hi';\nc();"
+
 test "multiple invocations of the compiler are separate" ->
   esl "(macro what (lambda () (return 'hi)))"
   esl "(what)"