separate let/set defaults by fallible/infallible status

[andrew4328]

(x, y) = (1, 2)
# is infallible so set is the default

match (1, 2) {
   (x, y) => ();
   # is fallible, so let is the default
}

currently this is aligned with most common patterns, but really they should be separated cleanly for maintenance sanity.