Implement Labeled Tuples

[WondAli]

Goal: Implement Labeled Tuple functionality into Hazel.
Note: I don't think I can change the source branch on the old pull request. This redesign of the implementation was started on a new branch, so the old pull request (and branch) is outdated.

TODO: Add more detail to the pull request; add more detail about syntax and semantics

What are Labeled Tuples?
An element in a tuple can be "labeled" or "named" with the = operator. For example, (x=1, y=2) is a tuple expression in which its elements are labeled x and y respectively. These labels are relevant only within their nearest enclosing tuple.
The = operator can be thought of as a binary operator that takes in a string and an expression, and returns the expression.

(Uncertain) A labeled element is type-consistent with the type of the element it is labeling.

Labeled Tuple Features
When analyzed against a labeled tuple type, the labeled items can be placed anywhere in the tuple expression and tuple type (the unlabeled items must be in relative order to each other).

• Ex: (x=1, True, y=6) is properly typed against (Bool, y=Int, x=Int)
• Ex: Let x=a, b = (y=2, x=4) will bind 4 to a and 2 to b

Dot Operators
The dot operator . can be applied to a tuple to extract a labeled element.
For example, (x=1, 2, 3).x is equivalent to 1
Currently undefined for anything except tuple expressions.

Implementation Strategy

1. Implement a "label" term to express the labeled tuple element.
2. Modify tuple semantics to consider labeled term contained within the tuple.
3. Implement the dot operator as a new expression that takes an expression and a pattern (var only)
4. Modify labeled tuple semantic interactions with let-binding, casting, function application, etc.

Checkpoints:

• finish writing the draft pull request
• Implement the Label term that simply wraps an element with a string.
• Tuples type-check assuming all elements are in order.
• Tuples type-check considering rearrangements of labeled elements.
• Tuple type-checking when there is an unequal number of labeled elements
• Dot operation on Tuple expressions
• Labeled tuples as function arguments
• Code Cleaning and Optimization
• Add documentation buffer for labeled tuples
  Known Bugs:
• Label uniqueness in a tuple needs to be checked during statics (currently dynamic check is broken too?)

[cyrus-]

@WondAli playing with this branch, looks like the labels aren't showing up in the types of the tuples (and so you get weird run-time errors when projecting out a label that doesn't exist). is there a specific issue with adding labels to product types?

[7h3kk1d]

TODO test nested dot operators