Fix approve max amount

[hippocampus-web3]

FIx for asgardex/asgardex-desktop#790

[dynst]

This is a follow-up to #1424 and #1434, right? Besides prepareApprove, do these lines in approve (which calls prepareApprove before these lines) need changing too?

xchainjs-lib/packages/xchain-evm/src/clients/client.ts

Lines 854 to 855 in 8c58393

	tx.gasLimit = BigInt(gasLimit.toString())
	tx.gasPrice = BigInt(gasPrice.toString())

And a line in transfer.

xchainjs-lib/packages/xchain-evm/src/clients/client.ts

Line 796 in 7ee4f15

tx.gasLimit = BigInt(txGasLimit.toString())

And it'll be a major API break, but hopefully #1406 wipes away all these BigNumber-to-BigInt conversions someday.

[hippocampus-web3]

At some point, we could consider addressing the change from bignumber.js, but it's so deeply integrated into the library that I'm not sure the time spent would be worth the benefit gained