Add some String class level documentation with examples

packages/builtin/string.pony

@@ -6,7 +6,44 @@ use @strtod[F64](nptr: Pointer[U8] box, endptr: USize)

class val String is (Seq[U8] & Comparable[String box] & Stringable)
  """
  A String is an ordered collection of characters.

  Strings don't specify an encoding.

  Example usage of some common String methods:

```pony
actor Main
  new create(env: Env) =>
    try
      // construct a new string
      let str = "Hello"

      // make an uppercased version
      let str_upper = str.upper()
      // make a reversed version
      let str_reversed = str.reverse()

      // add " world" to the end of our original string
      let str_new = str.add(" world")

      // count occurrences of letter "l"
      let count = str_new.count("l")

      // find first occurrence of letter "w"
      let first_w = str_new.find("w")
      // find first occurrence of letter "d"
      let first_d = str_new.find("d")

      // get substring capturing "world"
      let substr = str_new.substring(first_w, first_d+1)
      // clone substring
      let substr_clone = substr.clone()

      // print our substr
      env.out.print(consume substr)
  end
```
  """
  var _size: USize
  var _alloc: USize