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

[Paul_Cantrell]

IMO, yield is also the wrong keyword to use in case the language ever wants to support coroutines. However, I would support exactly this approach using some other keyword.

I suggested result as a placeholder up above, which has its own obvious problems:

@Paul_Cantrell

It's clear to me that return is the wrong keyword for if-/case-expressions, but for the example above, by the language's own precedents, a solution with the following general shape ought to be possible (using result as a placeholder keyword):

let message = if let thinger = fetchThinger() {
    "Found \(thinger.name)"
} else {
    log("Thinger fetch failed")
    result "ERROR: no thinger available"
}

…but I do think a resolution is lurking in this design space. And we could (I think?) use a word like result that isn't currently a reserved word, since the context in which this word would exist is not currently syntactically valid.

Although…hmm, accepting this proposal with only single-statement if expressions would negate what I just wrote: result or whatever keyword we ultimately want would appear as an identifier inside legal single-statement if expressions, making the multi-statement migration a breaking change. Yikes. Perhaps another strike against accepting the single-statement flavor as a stepping stone.