Fix bug where lambda required an application

src/parser.js

 
     return new AST.Lambda({
       parameters: parameters,
-      body: this.form(),
+      body: this.expr(),
     })
   }
 

test/parserTest.js

     }),
   )
 })
+
+test('allow returning atomic value from lambda', t => {
+  t.plan(1)
+  const tree = helpers.parse('(lambda (x) x)')
+
+  t.deepEqual(tree, [
+    new AST.Lambda({
+      parameters: [new AST.Identifier({ name: 'x' })],
+      body: new AST.Identifier({ name: 'x', line: 1 }),
+    }),
+  ])
+})