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.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"crypto/rand"
 	"os"
 )
 
@@ -20,4 +21,26 @@ func main() {
 	for _, arg := range os.Args[1:] {
 		println("arg:", arg)
 	}
+
+	// Check for crypto/rand support.
+	checkRand()
+}
+
+func checkRand() {
+	buf := make([]byte, 500)
+	n, err := rand.Read(buf)
+	if n != len(buf) || err != nil {
+		println("could not read random numbers:", err)
+	}
+
+	// Very simple test that random numbers are at least somewhat random.
+	sum := 0
+	for _, b := range buf {
+		sum += int(b)
+	}
+	if sum < 95*len(buf) || sum > 159*len(buf) {
+		println("random numbers don't seem that random, the average byte is", sum/len(buf))
+	} else {
+		println("random number check was successful")
+	}
 }

testdata/env.txt

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