Add optimization: foo["bar"] ==> foo.bar

[rspivak]

The feature was implemented in the following commits: https://github.com/rspivak/slimit/commit/d76d004b6c44a1a7d28719a6bf4db221b3dfc089 https://github.com/rspivak/slimit/commit/8d4f1a6cd8b263ffbdb4681fd50c2e4f59228752