add switch statement

[John Stilley]

We need a switch or elif functionality, to make the language more flexible. It might just be syntactic sugar. But it's SUPER common, and save from a lot of multiple embedding.

[John Stilley]

(switch 1.0 "first" 2.0 "second" default "last")

(switch
    1.0 "first"
    2.0 "second"
    default "last")