Parse function declarations without equals sign

[MeAmAnUsername]

Summary
Parse function declarations with a missing equals sign between the signature and the implementation.

Todo

• Decide whether to allow omitting equals signs
• Add tests
• Implement
• If allowed: add to documentation

Reason
Reason: many languages (including Java) do not have the =, so I expect many people new to PIE will not expect it to be required.

Description
Parse things like

func foo() -> int {
  9
}

This should either give an error (missing =) or be accepted as valid as well.
Currently thinking it is best not to accept it, as the current syntax makes it more clear what is actually happening.

Implementation
To allow omitting the equals sign, make = optional in the syntax and give an error in Statix analysis if it is not included unless the implementation matches PieFuncImpl(Block(_))

Search terms
parse, syntax, grammar, permissive, optional, omit, missing

Related issues

• Other issue for making grammar more permissive: More permissive parsing of foreign java functions/tasks/constructors #215, Replace rejections of keywords with errors #85