Support more characters for initializing a `BigInt` from a string

[konsumlamm]

Analogous to - for negative numbers, we could also support a leading +, which would be ignored. Another thing that would be nice is supporting _ (which also get ignored), akin to normal int literals:

let a = initBigInt("+10_000_000_000")

[dlesnoff]

We could also change the toString function. I would add optional parameters to toString to add _ and sign to the normal representation. By default the default parameters would not output them, so we can always get the number of digits in the specified radix representation by asking for len(number).