RFC: Extend pattern syntax

[rust-highfive]

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Tracking issue for postponed PR #99

[Centril]

Current status:

• Allow for parenthesisation of patterns.
• Change @-patterns to allow a pattern on either side.
• Add alternation (|) to patterns properly.
• Add pattern guards (pattern if condition) to patterns properly.
• Treat repeated variables as implicit guards for equality.
• Change the syntax of match-expressions/-statements to take a normal pattern.
  • Or-patterns part done
  • Pattern guards done
• ! patterns (Extend Patterns #2150)