Fix BN and rlp re-exports' types

Author: alcuadrado

src/externals.ts

@@ -4,8 +4,11 @@
  * @packageDocumentation
  */
 
-import * as BN from 'bn.js'
-import * as rlp from 'rlp'
+// TODO: This can be replaced with a normal ESM import once
+// the new major version of the typescript config package
+// is released and adopted here.
+import BN = require('bn.js');
+import rlp = require('rlp');
 
 /**
  * [`BN`](https://github.com/indutny/bn.js)