[SECURITY] Timing leaks in lib/native/bn.js

* Branching timing leaks
* https://github.com/bcoin-org/bcrypto/blob/b73dbc69884ecc790864e8ac513d627cdb0318d7/lib/native/bn.js#L3556
* https://github.com/bcoin-org/bcrypto/blob/b73dbc69884ecc790864e8ac513d627cdb0318d7/lib/native/bn.js#L3435
* powm leaks the value of `e`
* https://github.com/bcoin-org/bcrypto/blob/b73dbc69884ecc790864e8ac513d627cdb0318d7/lib/native/bn.js#L3869-L3897
* https://github.com/bcoin-org/bcrypto/blob/b73dbc69884ecc790864e8ac513d627cdb0318d7/lib/native/bn.js#L3889-L3893
* (This also breaks any modular inversion that relies on [`fermat()`](https://github.com/bcoin-org/bcrypto/blob/b73dbc69884ecc790864e8ac513d627cdb0318d7/lib/native/bn.js#L3812))

I wrote a library that implements [constant-time algorithms in TypeScript](https://github.com/soatok/constant-time-js) if you want to mitigate these risks.

Further reading: [[1]](https://soatok.blog/2020/08/27/soatoks-guide-to-side-channel-attacks/) [[2]](https://github.com/veorq/cryptocoding)

Start in lib/native/bn.js at the branching locations around lines 3435 and 3556, then inspect powm around lines 3869-3897 and fermat around line 3812. Compare the affected operations with the linked constant-time TypeScript library and ensure the reported timing leaks, including modular inversion through fermat(), are addressed.

Written by the indexing model from the issue text.