Move Rip::Nodes::Invocation#{object,callable}
Refactor standard invocation test to be much more thorough
Refactor Rip::Compiler::Lexer#normalize_word to always return Rip::WordToken
Normalize single '=' tokens into Rip::AssignmentToken
Extract standard invocation tests into a shared example to facilitate sharing with operator invocation
Refactor operator invocation tests to use shared invocation test (still failing)
Typo fixes