Go-style interfaces for Gerbil

[vyzo]

This adds an :std/interface library module that implements Go-style interfaces.
Interface instances package an object with its method implementations flat, so that we can dispatch without lookup.

TBD:

• unit test
• docs

Here is an example:

(interface Operation
  (start!)
  (apply x)
  (finish!))

(defstruct LinearAccumulator (state a b))

(defmethod {apply LinearAccumulator}
  (lambda (self x)
    (with ((LinearAccumulator state a b) self)
      (set! (LinearAccumulator-state self)
         (+ state (* a x) b)))))

(defmethod {start! LinearAccumulator}
  (lambda (self)
    (set! (LinearAccumulator-state self) 0)))

(defmethod {finish! LinearAccumulator}
  LinearAccumulator-state)


;; implementation using vanilla method dispatch
(def (fold-accumulator/vanilla acc iters)
  {start! acc}
  (for (x (in-range iters))
    {apply acc x})
  {finish! acc})

;; implementation using the Operation interface
(def (fold-accumulator/interface acc iters)
  (let (op (Operation acc))
    (Operation-start! op)
    (for (x (in-range iters))
      (Operation-apply op x))
    (Operation-finish! op)))

And here are some timings for 1M iterations, after compiling the above code:

> (import :tmp/interface-example)
> (def acc (LinearAccumulator #f 2 3))
> (time (fold-accumulator/vanilla acc 1000000))
(time (tmp/interface-example#fold-accumulator/vanilla acc (##quote 1000000)))
    0.046875 secs real time
    0.045979 secs cpu time (0.045979 user, 0.000000 system)
    no collections
    no bytes allocated
    no minor faults
    no major faults
1000002000000
> (time (fold-accumulator/interface acc 1000000))
(time (tmp/interface-example#fold-accumulator/interface acc (##quote 1000000)))
    0.019163 secs real time
    0.018647 secs cpu time (0.018647 user, 0.000000 system)
    no collections
    1952 bytes allocated
    no minor faults
    no major faults
1000002000000
> (time (fold-accumulator/interface acc 1000000))
(time (tmp/interface-example#fold-accumulator/interface acc (##quote 1000000)))
    0.017777 secs real time
    0.017372 secs cpu time (0.017372 user, 0.000000 system)
    no collections
    1072 bytes allocated
    no minor faults
    no major faults
1000002000000

[ober]

lgtm

[vyzo]

Fixed some edge cases with default arguments from mixins, will merge once it's green.