Use asn1js.Integer.fromBigInt instead of asn1js.Integer

[SWGovernikus]

The r and s values returned by the WebCryptoAPI are assigned as byte buffers to the asn1js.Integer class (new asn1js.Integer({ valueHex: rBuffer });).
The WebCryptoAPI does not apply any formatting to these values; it outputs byte arrays whose lengths correspond to the bit length (order) of the elliptic curve. As a result, smaller numerical values are left-padded with 0x00 bytes, which can lead to invalid or misinterpreted ASN.1 encodings. See asn1IntTest.spec.ts.
As a solution we propose to convert to BigInt and use the asn1js.Integer.fromBigInt method.

This should solve #385

[microshine]

Thanks for the fix, looks correct and should resolve the P‑521 encoding issue. I left a few small remarks in the review.

[SWGovernikus]

@microshine i hope the new test works. If there is anything else to fix, please let me know. :)

[coveralls]

coverage: 74.554% (+0.02%) from 74.539%
when pulling 7983d0d on Governikus:master
into 82ece66 on PeculiarVentures:master.

[gtjarks]

Hello @microshine,
is there anything else needed for the ticket, or are there still adjustments to be made to the code?

[SWGovernikus]

Hello @microshine , sorry to bother you :)
Please let me know if anything can be done on my side to merge this. Thank You!