Fix top-level tuple declarations

[JSAbrahams]

Relevant issues

Closes #253

Ideally should've been part of release v0.3.1, but I forgot about this.

Summary

• Add TupleLiteral construct to core language
• Don't annotate tuples with types in Python
• Treat top-level declarations of variabls as tuples
• More descriptive error message when class variable missing type annotation

In short, we get:

def (a, b) := (20, 20)

class MyClass:
    def (a, b): (Int, Int) := (20, 20)

print(MyClass().a)
print(MyClass().b)

Added Tests

• [x] Top-level tuple declaration in class
• Top-level tuple declarations (collection/tuple.mamba)
• Unhappy Tuple assigning to itself, to check the env does not pollute right side of assignment

[codecov[bot]]

Codecov Report

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