Generate captcha representations deterministically.

WARNING: introduces API incompatibility!

This package generates captcha representations on-the-fly; for instance,
if captcha solution was "123456", every call to NewImage() using this
sequence of digits would generate a different random image containing
"123456"; similarly, NewAudio() would generate a different audio
pronouncing the same sequence: 1, 2, 3, 4, 5, 6.

If a user, instead of storing generated outputs, exposes this
functionality from their server, which is the default and recommended
behaviour, an attacker could try loading the same image or audio over
and over again in attempt to arrive at the most correct optical/voice
recognition result.

Instead of using a global non-deterministic pseudorandom number
generator to distort images and audio, this commit introduces a
deterministic PRNG for each image/audio. This PRNG uses a combination of
a global secret key (generated once during initialization from a system
CSPRNG) and captcha id and solution to produce pseudorandom numbers for
each representation deterministically. Thus, calling NewImage() with the
same captcha id and solution at different times will result in the same
image (ditto for NewAudio).

To make results unique not only for different solutions, but also for
ids, these incompatible changes to public API have been introduced:

NewImage and NewAudio changed from:

  func NewImage(digits []byte, width, height int) *Image
  func NewAudio(digits []byte, lang string) *Audio

to:

  func NewImage(id string, digits []byte, width, height int) *Image
  func NewAudio(id string, digits []byte, lang string) *Audio

That is, they now accept an additional captcha `id` argument.
No other interfaces changed.

Described changes also improved performance of generating captchas.