Buffer operator collects items into a user-defined buffer,
which is emitted when the buffer is full or when a given
time interval has elapsed.

Example:

    var src Observable[string] // Some stream of strings

    sliceBuffered := stream.Buffer[[]string, string](
      src,
      16, // buffer size
      time.Second, // wait up to 1 second for 16 items
      func(buf []string, s string) []string {
        return append(buf, s)
      })

    mapBuffered := stream.Buffer[map[string]struct{}, string](
      src,
      16, // buffer size
      time.Second, // wait up to 1 second for 16 items
      func(buf map[string]struct{}, s string) map[string]struct{} {
        if buf == nil {
          return map[string]struct{}{s: struct{}{}}
        }
        buf[s] = struct{}{}
        return buf
      })

Signed-off-by: Jussi Maki <jussi@isovalent.com>