BN re-export failure

[martriay]

Hello! I'm not being able to use the reexported version of BN.js from TypeScript due to how the export is defined. I'm using the 7.0.3 release.

Example code:

import { BN } from 'ethereumjs-util';

function hexStringIsZero(hexNumber: string): boolean {
  return new BN(hexNumber, 'hex').isZero();
}

Printed error plus suggested fix:

../core/src/Foo.ts:51:14 - error TS2351: This expression is not constructable.
  Type 'typeof BN' has no construct signatures.

51   return new BN(hexNumber, 'hex').isZero();
                ~~

  ../core/node_modules/ethereumjs-util/dist/externals.d.ts:6:1
    6 import * as BN from 'bn.js';
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Type originates at this import. A namespace-style import cannot be called or constructed, and will cause a failure at runtime. Consider using a default import or import require here instead.

[alcuadrado]

Thanks for reporting this, @martriay! I fixed it in #270.