Support an alternate for syntax that calls a higher-order function

The last argument of the call must be a block, and the type of this
argument must a function returning bool. `break` and `cont` are
supported in the body of the block, and return `false` or `true` from
the function. When the end of the function is reached, `true` is
implicitly returned.

    for vec::all([1, 2, 3]) {|elt|
        if elt == 2 { break; }
        log(error, elt);
    }

Issue #1619

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