(ruby-use-smie): New var.
(ruby-smie-grammar): New constant.
(ruby-smie--bosp, ruby-smie--implicit-semi-p)
(ruby-smie--forward-token, ruby-smie--backward-token)
(ruby-smie-rules): New functions.
(ruby-mode-variables): Setup SMIE if applicable.
* test/indent/ruby.rb: Fix indentation after =; add more cases.