Comparisons with large exponents are extremely slow

[DanielBauman88]

        let a = BigDecimal::from_str("1e99999999999999").unwrap();
        let zero = BigDecimal::zero();
        let bool = a == zero;

I haven't spent a lot of time looking into what's happening here, but it looks like the lib is getting stuck in ten_to_the

I'd expect BigDecimal to function in reasonable time, or to throw errors if there are values it can't process efficiently. I wouldn't expect it to hang on processing valid values.

I'm running into some of these changes by using BigDecimal to parse some JSON inputs which aren't under my control - which I'm going to work on avoiding in future.

[akubera]

Looking into this raises (in my opinion) another issue: when debugging "big" decimals, the debugger will attempt to stringify the entire decimal.

This is related to #108, where lack of exponential syntax might panic.

Regardless: this can be fixed without the planned reimplementation. I think I can knock that out quickly.

[akubera]

Going for a double on this one:

I was hoping to optimize ten_to_the as well as PartialEq::eq.

As there often is, I have two competing algorithms. I used criterion to benchmark different values of n (in 10^n) and it looks like there's a clear point to switch over:

By my calculations (on my computer) it's about n=1410 (i.e. 10^1410).

Unfortunately, they're both obviously climbing rapidly, so I'm going to check a few more things.

The primary issue is fixed, I will squash commits and push soon. It requires allocations to compare decimals digit-by-digit, which will be a lot more efficient than the old code (written ~7 years ago!).

This was fun, thanks.