Annotated sequence and finite range (RFC)

[countvajhula]

This is just a sample to address #27 and start to address #30 . It needs feedback from you @lexi-lambda .

[countvajhula]

@lexi-lambda Depending on your thoughts re: the issues I raised, I'll probably solicit help from the community in creating PRs, since I've had these on my todo list for quite a while and haven't gotten to them yet. Nevertheless, I am keen on doing my part to at least shepherd any fixes and improvements to streamline the use of generic collections in Racket -- the only thing I'd need from you is your input on whether these fixes make sense or if you have other ideas, and I can take point on moving them forward. Thanks!