crypto/rand: replace this package with a TinyGo version

This package provides access to an operating system resource
(cryptographic numbers) and so needs to be replaced with a TinyGo
version that does this in a different way.

I've made the following choices while adding this feature:

  - I'm using the getentropy call whenever possible (most POSIX like
    systems), because it is easier to use and more reliable. Linux is
    the exception: it only added getentropy relatively recently.
  - I've left bare-metal implementations to a future patch. This because
    it's hard to reliably get cryptographically secure random numbers on
    embedded devices: most devices do not have a hardware PRNG for this
    purpose.

testdata/env.txt

@@ -3,3 +3,4 @@ ENV2: VALUE2
 
 arg: first
 arg: second
+random number check was successful