Fix bug in big.int.Mutable.toManaged() and add tests

Fixes #5918

lib/std/math/big/int.zig

@@ -99,7 +99,7 @@ pub const Mutable = struct {
     pub fn toManaged(self: Mutable, allocator: *Allocator) Managed {
         return .{
             .allocator = allocator,
-            .limbs = limbs,
+            .limbs = self.limbs,
             .metadata = if (self.positive)
                 self.len & ~Managed.sign_bit
             else

lib/std/math/big/int_test.zig

@@ -2,6 +2,7 @@ const std = @import("../../std.zig");
 const mem = std.mem;
 const testing = std.testing;
 const Managed = std.math.big.int.Managed;
+const Mutable = std.math.big.int.Mutable;
 const Limb = std.math.big.Limb;
 const DoubleLimb = std.math.big.DoubleLimb;
 const maxInt = std.math.maxInt;
@@ -1453,3 +1454,24 @@ test "big.int gcd one large" {
 
     testing.expect((try r.to(u64)) == 1);
 }
+
+test "big.int mutable to managed" {
+    const allocator = testing.allocator;
+    var limbs_buf = try allocator.alloc(Limb, 8);
+    defer allocator.free(limbs_buf);
+
+    var a = Mutable.init(limbs_buf, 0xdeadbeef);
+    var a_managed = a.toManaged(allocator);
+
+    testing.expect(a.toConst().eq(a_managed.toConst()));
+}
+
+test "big.int const to managed" {
+    var a = try Managed.initSet(testing.allocator, 123423453456);
+    defer a.deinit();
+
+    var b = try a.toConst().toManaged(testing.allocator);
+    defer b.deinit();
+
+    testing.expect(a.toConst().eq(b.toConst()));
+}