LibCrypto: Replace {Unsigned,Signed}BigInteger impl with LibTomMath

Replace the implementation of maths in `UnsignedBigInteger`
and `SignedBigInteger` with LibTomMath. This gives benefits in terms of
less code to maintain, correctness and speed.

These changes also remove now-unsued methods and improve the error
propagation for functions allocating lots of memory. Additionally, the
new implementation is always trimmed and won't have dangling zeros when
exporting it.

Libraries/LibCrypto/BigFraction/BigFraction.cpp

@@ -157,14 +157,14 @@ double BigFraction::to_double() const
     // NOTE: the precision of the result will be 63 bits (more than 53 bits necessary for the mantissa of a double).
     if (top_bit_numerator < (top_bit_denominator + 64)) {
         shift_left_numerator = top_bit_denominator + 64 - top_bit_numerator;
-        numerator = numerator.shift_left(shift_left_numerator); // copy
+        numerator = MUST(numerator.shift_left(shift_left_numerator)); // copy
     }
     // NOTE: Do nothing if numerator already has more than 64 bits more than denominator.
 
     // 2. Divide [potentially shifted] numerator by the denominator.
     auto division_result = numerator.divided_by(denominator);
     if (!division_result.remainder.is_zero()) {
-        division_result.quotient = division_result.quotient.shift_left(1).plus(1); // Extend the quotient with a "fake 1".
+        division_result.quotient = MUST(division_result.quotient.shift_left(1)).plus(1); // Extend the quotient with a "fake 1".
         //  NOTE: Since the quotient has at least 63 bits, this will only affect the mantissa
         //        on rounding, and have the same effect on rounding as any fractional digits (from the remainder).
         shift_left_numerator++;