wasm: add support for the crypto/rand package

This is done via wasi-libc and the WASI interface, for ease of
maintenance (only one implementation for both WASI and JS/browsers).

testdata/rand.go

@@ -0,0 +1,24 @@
+package main
+
+import "crypto/rand"
+
+// TODO: make this a test in the crypto/rand package.
+
+func main() {
+	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")
+	}
+}