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/crypto.odin

@@ -39,3 +39,14 @@ compare_byte_ptrs_constant_time :: proc "contextless" (a, b: ^byte, n: int) -> i
 	// iff v == 0, setting the sign-bit, which gets returned.
 	return int((u32(v)-1) >> 31)
 }
+
+// rand_bytes fills the dst buffer with cryptographic entropy taken from
+// the system entropy source.  This routine will block if the system entropy
+// source is not ready yet.  All system entropy source failures are treated
+// as catastrophic, resulting in a panic.
+rand_bytes :: proc (dst: []byte) {
+	// zero-fill the buffer first
+	mem.zero_explicit(raw_data(dst), len(dst))
+
+	_rand_bytes(dst)
+}