Add bigint support

[wemeetagain]

The logic for number encoding works fine for bigints.

• Add bigint to Input type
• Add typeof check to allow bigint encoding to share number encoding code path
• Add simple unit test to verify bigint encoding

[alcuadrado]

Hey @wemeetagain! Thanks for this contribution.

While this is backward compatible, its tests won't run in Node 8. We are currently trying to define a policy about which versions of Node to support, so we may not be able to merge it just yet. You can read more about it here.

[wemeetagain]

Okydoky, it looks like the consensus is to wait until Node 8 goes out of maintenance (Dec 31) to put any sort of policy in motion?

Feel free to close / ignore this PR until the time is right.

[axic]

@alcuadrado would it make sense to support some kind of shim for this?

[alcuadrado]

AFAIK there's no way to shim native bigints. Maybe a partial shim can be implemented for this tests though.

[coveralls]

Coverage remained the same at 87.654% when pulling d1138f9 on ChainSafe:add-bigint into f8394b3 on ethereumjs:master.

[wemeetagain]

bump :)
I added a switch to skip the BigInt test in CI for older versions of node.

[holgerd77]

Sorry, my bigint experience is pretty limited. Won't this cause major problems for people who want to use this library within a browser context?

[wemeetagain]

This change is meant to be backwards compatible. The only change to the generated js should be the additional typeof check, a condition which will just return false for browsers without bigint support.

The intToHex and intToBuffer functions work without modifications with both number and bigint inputs, reason being the syntax for arithmetic operations is the same for number vs bigint.

[holgerd77]

Thanks @wemeetagain, this looks good, will merge now.

Is this just generally useful contribution, or do you need this as some basis for further work and should we do a release (patch level I would guess?) on this?