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).

tests/core/crypto/test_core_crypto.odin

@@ -120,6 +120,7 @@ main :: proc() {
     test_poly1305(&t)
     test_chacha20poly1305(&t)
     test_x25519(&t)
+    test_rand_bytes(&t)
 
     bench_modern(&t)
 

tests/core/crypto/test_core_crypto_modern.odin

@@ -4,6 +4,7 @@ import "core:testing"
 import "core:fmt"
 import "core:mem"
 import "core:time"
+import "core:crypto"
 
 import "core:crypto/chacha20"
 import "core:crypto/chacha20poly1305"
@@ -303,6 +304,45 @@ test_x25519 :: proc(t: ^testing.T) {
     // how to work with JSON.
 }
 
+@(test)
+test_rand_bytes :: proc(t: ^testing.T) {
+	log(t, "Testing rand_bytes")
+
+	if ODIN_OS != "linux" {
+		log(t, "rand_bytes not supported - skipping")
+		return
+	}
+
+	allocator := context.allocator
+
+	buf := make([]byte, 1 << 25, allocator)
+	defer delete(buf)
+
+	// Testing a CSPRNG for correctness is incredibly involved and
+	// beyond the scope of an implementation that offloads
+	// responsibility for correctness to the OS.
+	//
+	// Just attempt to randomize a sufficiently large buffer, where
+	// sufficiently large is:
+	//  * Larger than the maximum getentropy request size (256 bytes).
+	//  * Larger than the maximum getrandom request size (2^25 - 1 bytes).
+	//
+	// While theoretically non-deterministic, if this fails, chances
+	// are the CSPRNG is busted.
+	seems_ok := false
+	for i := 0; i < 256; i = i + 1 {
+		mem.zero_explicit(raw_data(buf), len(buf))
+		crypto.rand_bytes(buf)
+
+		if buf[0] != 0 && buf[len(buf)-1] != 0 {
+			seems_ok = true
+			break
+		}
+	}
+
+	expect(t, seems_ok, "Expected to randomize the head and tail of the buffer within a handful of attempts")
+}
+
 @(test)
 bench_modern :: proc(t: ^testing.T) {
 	fmt.println("Starting benchmarks:")