Allow multiple constraint arrows (=>) in type scheme syntax

[brianhuffman]

This would be convenient when writing large type signatures that contain many complex type constraints. It would let us write each constraint on a separate line with its own => at the end, making it much easier to cut/paste/rearrange the constraints.

For example, we could write

ExpandKey :
  {KEY_NUM_COLS, NUM_ROUNDS, NUM_KEY_EXP_ROUNDS}
  (8 >= KEY_NUM_COLS) =>
  (KEY_NUM_COLS >= 2) =>
  fin NUM_ROUNDS =>
  ((1 + NUM_KEY_EXP_ROUNDS) * KEY_NUM_COLS >= 4 * (1 + NUM_ROUNDS)) =>
  8 >= width NUM_KEY_EXP_ROUNDS =>
  (NUM_KEY_EXP_ROUNDS >= 1) =>
  { key : [KEY_NUM_COLS][4]byte_t } ->
  { roundKeys : [NUM_ROUNDS+1]roundKey_t }

instead of something like

ExpandKey :
  {KEY_NUM_COLS, NUM_ROUNDS, NUM_KEY_EXP_ROUNDS}
  (8 >= KEY_NUM_COLS, KEY_NUM_COLS >= 2,
   fin NUM_ROUNDS,
   (1 + NUM_KEY_EXP_ROUNDS) * KEY_NUM_COLS >= 4 * (1 + NUM_ROUNDS),
   8 >= width NUM_KEY_EXP_ROUNDS, NUM_KEY_EXP_ROUNDS >= 1) =>
  { key : [KEY_NUM_COLS][4]byte_t } ->
  { roundKeys : [NUM_ROUNDS+1]roundKey_t }

It could be treated as syntactic sugar for a comma-separated list of constraints. When pretty-printing type schemas, we could just always use the comma-separated syntax that we have now.