Use genuine BN type

[vshab]

Description

Don't wrap BN but use type from BN library to improve compatibility with BN operations. Without this change this code is invalid:

const bigNumber = new BN(3);

fromWei(bigNumber.add(new BN(100)));

Fixes #2350

Type of change

• Bug fix

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no warnings.
• I have updated or added types for all modules I've changed
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run test in the root folder with success and extended the tests if necessary.
• I ran npm run build in the root folder and tested it in the browser and with node.
• I ran npm run dtslint in the root folder and tested that all my types are correct
• I have tested my code on an ethereum test network.

[coveralls]

Coverage decreased (-0.009%) to 98.672% when pulling b1f49a5 on vshab:fix/bn-type into 22a190c on ethereum:1.0.

[vshab]

@nivida CI fixed