Add a lazy string builder

[TimWhiting]

Hi @daanx,

Wanted to have some fun this weekend, and try out the new lazy constructors.

Ended up making this lazy string builder, partly inspired by your pretty print module.
Conversions / additions to string builder for polymorphic types are made using show for the type - but lazily.
You can always create a small method for any type to build it in a particular way, and use these basic building blocks just for combining.

I understand if this is not something you want in std/core, but I wanted to share.

[TimWhiting]

The sample code here:

Using |-| for spaces |.| for direct appending, \ for newline and \- for newline + indent.

import std/core/stringb
import std/num/float64

fun sb(l: list<int>)
  match l
    Nil -> build()
    Cons(x, Nil) ->
      x.build()
    Cons(x, xs) ->
      x |-| sb(xs)

fun tupled(l: list<a>, ?show: a -> string)
  "(" |.| tuprec(l) |.| ")".build()

fun tuprec(l : list<a>, ?show: a -> string)
  match l
    Nil -> build()
    Cons(x, Nil) ->
      x.build()
    Cons(x, xs) ->
      x |.| "," |-| tuprec(xs)

fun main()
  [1, 2, 3].sb.println
  "".println

  val b = "Hello" |-| "world!" |.|
            indented([1, 2, 3].sb \ [1, 2, 3].build()) |.|
            2 \ 4.5.build()
  b.println
  "".println

  val b1 = "Hello" |-| "world!" 
              \- [1, 2, 3].sb 
                \- [1, 2, 3].tupled
                  \- 2 
                  \ 4.5.build()
  b1.println

prints the following

1 2 3

Hello world!
  1 2 3
  [1,2,3]
2
4.5

Hello world!
  1 2 3
    (1, 2, 3)
      2
      4.5

Conversions / additions to string builder for polymorphic types are made using show for the type - but lazily.