SE-0380: `if` and `switch` expressions

[codename]

Evolution idea: implicit return in single-line switch cases

This is something I have wished for ever since implicit returns were introduced. However, I see it as purely a generalization of the current rules for implicit returns, to allow branching using control flow statements, rather than just treating control flow blocks as expressions universally. IOW I’m not a huge fan of things like using a switch on the right hand of an assignment expression (let result = switch value { … }), but I do wish I could use implicit returns in control flow as a general…