Apply flambda2 switch table optimization to naked numbers and symbols

[mshinwell]

This ensures that the optimization that turns an flambda2 Switch into a lookup table, in certain circumstances, applies to the naked number kinds as well, e.g.

type t = A | B | C

let f1 = function
  | A -> 8
  | B -> 4
  | C -> 2

let f2 = function
  | A -> #8l
  | B -> #4l
  | C -> #2l

[mshinwell]

This also now deals with symbols, so it will work for things like boxed floats too.