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.

src/crypto/rand/rand_getentropy.go

@@ -0,0 +1,38 @@
+// +build darwin freebsd wasi
+
+// This implementation of crypto/rand uses the getentropy system call (available
+// on both MacOS and WASI) to generate random numbers.
+
+package rand
+
+import (
+	"errors"
+	"unsafe"
+)
+
+var errReadFailed = errors.New("rand: could not read random bytes")
+
+func init() {
+	Reader = &reader{}
+}
+
+type reader struct {
+}
+
+func (r *reader) Read(b []byte) (n int, err error) {
+	if len(b) != 0 {
+		if len(b) > 256 {
+			b = b[:256]
+		}
+		result := libc_getentropy(unsafe.Pointer(&b[0]), len(b))
+		if result < 0 {
+			// Maybe we should return a syscall.Errno here?
+			return 0, errReadFailed
+		}
+	}
+	return len(b), nil
+}
+
+// int getentropy(void *buf, size_t buflen);
+//export getentropy
+func libc_getentropy(buf unsafe.Pointer, buflen int) int