Allow mutable identifiers within tuples and top-level tuples in classes

[JSAbrahams]

Relevant issues

Fixes #163 Fixes #161

Summary

• Update parsing rules for tuple identifiers
• Extract identifiers from tuples when building classes for Context. A variable is mutable if either the tuple itself is mutable, or the the specific variable is mutable.

Added Tests

• Happy Access variables in class which are defined within a tuple within said class
• Happy Assign to tuple where only one variable is mutable
• Happy Variable within a mutable tuple is itself mutable
• Sad Reassign to an immutable variable defined within a tuple
• Sad Type annotation within a tuple