Use get_or_insert to add values to Option<Vec> fields

[rnestler]

This method was added in Rust 1.20.0. See: https://doc.rust-lang.org/std/option/enum.Option.html#method.get_or_insert

It simplifies the code a lot.

Checklist

• [x] Tests added if applicable
• [x] README updated if applicable
• [x] CHANGELOG updated if applicable
• [x] If a commit includes a breaking change, include the string [breaking-change] somewhere in the commit message

[rnestler]

Since this bumps the minimal required Rust version, is this considered a breaking change?

[dbrgn]

Nice, I like the change :)

Since this bumps the minimal required Rust version, is this considered a breaking change?

I'd say so, yes.

[rnestler]

@dbrgn Ping :smiley: