Implementing let blocks, adding non-regression test

[litchipi]

Implementation of let blocks in the form:

let foo = 3,
    bar : Num = 2,
    message = "Hello World",
    f = fun x => x,
    f2 : Num -> Bool = fun x => x == 2,
in

Adds a new node LetBlock in the AST that basically mimic all actions from simple Let term, simply looping through all the definitions and adding them to the environment before passing the environment to the final term.

Made for feature request #494
Waiting for PR #502 and #470 to be merged.
Requesting reviews on the regression test to be sure to cover edge cases.

[yannham]

The overall changes look reasonable (it misses the grammar but I imagine it's because it's still draft), but this doesn't implement the semantics of #494 that states let-blocks should be mutually recursive, as for record, and not just equivalent to a series of simple lets. See the corresponding comments.

Also, tagging @ysndr as this may need additional support in the LSP.

[yannham]

Same remark about cloning.

[yannham]

Same thing as with evaluation, you need to typecheck this in a recursive environment. You can look at the RecRecord case to see how to do it. We may even separate this typechecking of mutually recursive bindings in a separate helper as I expect the code to be almost the same for let blocks and recursive records.