Fixes issue #8

  let f x = x * x
  let f x = x ** x ** x
  let f x = +. x
all work

cold/ files built by running fan on main/ files

main/lex_fan:
  add "+." as keyword so it can be used as either a prefix or infix (like "-.")

main/parse_fan:
  add missing(?) keywords to the parser.
  make "**" right associative
  add trivial unary "+" rule where the unary minus one is (handles "let f x = +. x" for the only reason that OCaml already permits it)