Move multiple classes under grammar

[alitaso345]

Summary

• In Refactor states.rb and grammar.rb #50, multiple classes were separated from grammar.rb, but the file location was not ideal.
• Multiple classes have been moved to Lrama::Grammar from Lrama.

Changes Made

• Class files for Code, ErrorToken, Precedence, Printer, Reference, Rule, and Symbol have been relocated to under Lrama::Grammar.
• Class paths in grammar_spec.rb and parser_spec.rb have been updated to reflect this change.

[yui-knk]

LGTM, thank you!