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.
We are currently using Core::DateTime, which is meant to represent local
time. However, we are doing no conversion between the parsed time in UTC
and local time, so we end up comparing time stamps from different time
zones.
Instead, store the parsed times as UnixDateTime, which is UTC. Then we
can always compare the parsed times against the current UTC time.
This also lets us store parsed milliseconds.
The underlying reason is an unconditional call to consume(), even if
there is no reason to expect that the string continues.
This crash was discovered by OSS-Fuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=42354
This bug exists since the code was first written in April 2021:
13abbc5ea8
