LibCrypto+LibJS: Better bigint bitwise_and binop

Bitwise and is defined in terms of two's complement, so some converting
needs to happen for SignedBigInteger's sign/magnitude representation to
work out.

UnsignedBigInteger::bitwise_not() is repurposed to convert all
high-order zero bits to ones up to a limit, for the two's complement
conversion to work.

Fixes test262/test/language/expressions/bitwise-and/bigint.js.

Userland/Libraries/LibJS/Tests/builtins/BigInt/bigint-basic.js

@@ -37,8 +37,14 @@ describe("correct behavior", () => {
 
     test("bitwise operators", () => {
         expect(12n & 5n).toBe(4n);
+        expect(3n & -2n).toBe(2n);
+        expect(-3n & -2n).toBe(-4n);
+        expect(-3n & 2n).toBe(0n);
+
         expect(1n | 2n).toBe(3n);
+
         expect(5n ^ 3n).toBe(6n);
+
         expect(~1n).toBe(-2n);
         expect(~-1n).toBe(0n);