Make `os2.random_string` use `context.random_generator`

This removes the data race caused by multiple threads using the
unprotected global `random_string_seed`, so long as no two threads share
the same random generator; this is the default case.

Additionally, `os2.random_string` now takes into account the full buffer
slice given to it.

core/os/os2/internal_util.odin

@@ -3,6 +3,7 @@ package os2
 
 import "base:intrinsics"
 import "base:runtime"
+import "core:math/rand"
 
 
 // Splits pattern by the last wildcard "*", if it exists, and returns the prefix and suffix
@@ -84,45 +85,15 @@ concatenate :: proc(strings: []string, allocator: runtime.Allocator) -> (res: st
 	return string(buf), nil
 }
 
-
-
-@(private="file")
-random_string_seed: [2]u64
-
-@(init, private="file")
-init_random_string_seed :: proc() {
-	seed := u64(intrinsics.read_cycle_counter())
-	s := &random_string_seed
-	s[0] = 0
-	s[1] = (seed << 1) | 1
-	_ = next_random(s)
-	s[1] += seed
-	_ = next_random(s)
-}
-
-@(require_results)
-next_random :: proc(r: ^[2]u64) -> u64 {
-	old_state := r[0]
-	r[0] = old_state * 6364136223846793005 + (r[1]|1)
-	xor_shifted := (((old_state >> 59) + 5) ~ old_state) * 12605985483714917081
-	rot := (old_state >> 59)
-	return (xor_shifted >> rot) | (xor_shifted << ((-rot) & 63))
-}
-
 @(require_results)
 random_string :: proc(buf: []byte) -> string {
-	@(static, rodata) digits := "0123456789"
-
-	u := next_random(&random_string_seed)
-
-	b :: 10
-	i := len(buf)
-	for u >= b {
-		i -= 1
-		buf[i] = digits[u % b]
-		u /= b
+	for i := 0; i < len(buf); i += 16 {
+		n := rand.uint64()
+		end := min(i + 16, len(buf))
+		for j := i; j < end; j += 1 {
+			buf[j] = '0' + u8(n) % 10
+			n >>= 4
+		}
 	}
-	i -= 1
-	buf[i] = digits[u % b]
-	return string(buf[i:])
+	return string(buf)
 }