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/env.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"crypto/rand"
 	"os"
 )
 
@@ -21,26 +20,4 @@ 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

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

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")
+	}
+}

testdata/rand.txt

@@ -0,0 +1 @@
+random number check was successful