API: Rename `CallExpr::operand` -> `CallExpr::func`; `*::pattern` -> `*::pat`

[xFrednet]

Some smaller breaking changes before v0.4.0 probably best summarized by the changelog entry:

• Renamed CallExpr::operand to CallExpr::func
• Renamed RefPat::pattern() -> RefPat::pat()
• Renamed OrPat::patterns() -> RefPat::pats()

Yeah, and that's it :D

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I also moved the semver check higher in our CI file. I thought about making it the first item, but I feel like the others are still more important and should be the first visible CI checks. Having semver higher up is already a nice change :)

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Closes https://github.com/rust-marker/marker/issues/297

Closes https://github.com/rust-marker/marker/issues/295

[xFrednet]

Good call, I forgot about that. Thanks!