Add a .toBigInt() method to BigNumber type

[mds1]

Ethers accepts BigInt's as inputs for BigNumberish types, but getting a BigInt from a returned BigNumber is a bit clunky with BigInt(myBigNumber.toString())

I saw in #594 (comment) you mentioned that "There will likely be an instance method that will return a BigInt (similar to the instance.toNumber())", though that method still doesn't exist.

I'm happy to submit a PR for it, as long as you still support adding an instance.toBigInt() method, and my only question would be whether to throw or return null on platforms that don't support BigInt

[ricmoo]

I have a bunch of other changes I’m pushing out today with v5.1, and this is something that makes sense for that minor bump. I’ll add it today. :)

[ricmoo]

(thanks for the reminder!)

[mds1]

Awesome, thank you!

[ricmoo]

This should now be available in 5.1.0.

Try it out and let me know if there are any issues. :)

[mds1]

Looks like it's working great so far, appreciate the quick turnaround too!