Similar to the bitwise_and change, but we have to be careful to
sign-extend two's complement numbers only up to the highest set bit
in the positive number.
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.
Bitwise operators are defined on two's complement, but SignedBitInteger
uses sign-magnitude. Correctly convert between the two.
Let LibJS delegate to SignedBitInteger for bitwise_not, like it does
for all other bitwise_ operations on bigints.
No behavior change (LibJS is now the only client of
SignedBitInteger::bitwise_not()).
Step 12 was using `lhs.is_bigint()` instead of `rhs.is_bigint()`,
meaning you got:
```js
1n == Object(1n); // true
Object(1n) == 1n; // false
```
This also adds spec comments to is_loosely_equal.
