Set type verifier

[Earl-Wu]

• Added set type verifier: The user can now add Ocaml-style type declarations to their code, and the verifier will use program transformation + sato to find any potential type errors
• on_ast.ml: Added new constructors to the AST to include type declrations
• ton_to_on.ml: Program transformation code; this module will insert the corresponding checks to the natodefa code for sato to run checks on
• on_parser.mll and on_lexer.mly: Modified the lexer and parser to handle type declarations
• Added a couple of basic tests for the verifier

[kelvinqian00]

So I reviewed your pull requests and it has a number of merge conflicts. Some of them revolve around the fact that I reamed the type-checker executable to sato (and all associated .ml files), so you'd have to align the naming conventions. Others are due to the fact that I moved the test-sources directory to be inside test. So make sure that your tests work with the JSON-based test harness that I have in master (since I must've added that after you checked out your own branch).