crypto: Add rand_bytes

This adds `rand_bytes(dst: []byte)` which fills the destination buffer
with entropy from the cryptographic random number generator.  This takes
the "simple is best" approach and just directly returns the OS CSPRNG
output instead of doing anything fancy (a la OpenBSD's arc4random).

core/crypto/rand_generic.odin

@@ -0,0 +1,7 @@
+package crypto
+
+when ODIN_OS != "linux" {
+	_rand_bytes :: proc (dst: []byte) {
+		unimplemented("crypto: rand_bytes not supported on this OS")
+	}
+}