lang: add basic unpacking for union types

[zerbina]

Add the Match construct to the source language, which is used for accessing union elements.

A Match is an expression and must be exhaustive, with one and only one As handler present for each type in the union. Because borrowing and semantics for values aren't fully decided on at this point, the handlers always receive a copy of the union element, making it easier to change the semantics later on.

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To-Do

• wait for the testable meta-language-based source language definition to be available
• define the semantics for Match
• add tests for Match
• write a proper commit message

Notes For Reviewers

• I'm not set on Match and As -- suggestions for alternative names are welcome