vine: `fn` trait [fork/drop]

[tjjfvi]

• replace the current fn(A, B, C) -> D type with an fn F(A, B, C) -> D trait
• fn foo[T](x: X) -> Y { ... } creates:
  • a unique type fn foo[T]
  • const foo[T]: fn foo[T]
  • impl foo[T]: fn (fn foo[T])(X) -> D
• fn expressions create unique, unnameable type, with inherent implementation of the appropriate fn trait
  • can add +/?/* suffix to fn keyword to additionally implement Fork/Drop (see vine: Fork/Drop #171)
• TBD: how, exactly, do fn impl items work?

fn map[T, U, F; F*, fn F(T) -> I](list: List[T], f: F) -> List[U] { ... }

struct MulBy(N32);

mod MulBy {
  pub impl call: fn MulBy(N32) -> N32 {
    fn call(MulBy(a): N32, b: N32) -> N32 {
      a * b
    }
  }
}

fn curry_mul(a: N32) -> MulBy {
  MulBy(a)
}

curry_mul(2)(23) // 46