Support OxCaml unboxed types

[shonfeder]

Unboxed tuples and records are already supported. Unboxed named types are all missing the #:

type pt = { x : int; y : int }
type line = { p : pt#; q : pt# }

type unboxed = {
  float_unboxed: float#; 
  int64_unboxed: int64#;
  float32_unboxed: float32#;
  int8_unboxed: int8#;
  int16_unboxed: int16#;
  char_unboxed: char#;
}

type simd = {
  int8x16_unboxed : int8x16#;
  int16x8_unboxed : int16x8#;
  int32x4_unboxed : int32x4#;
  int64x2_unboxed : int64x2#;
  float32x4_unboxed : float32x4#;
  float64x2_unboxed : float64x2#;
  int8x32_unboxed : int8x32#;
  int16x16_unboxed : int16x16#;
  int32x8_unboxed : int32x8#;
  int64x4_unboxed : int64x4#;
  float32x8_unboxed : float32x8#;
  float64x4_unboxed : float64x4#;
}

Specified by @art-w

PR: #1407