Use big integers for ASN.1 `INTEGER`s

[ydahhrk]

ASN.1 INTEGERs are unbounded, but Fort parses them into primitives.

It looks like this happens several times in the code:

• SignedData version
• SignerInfo version
• Manifest version
• ROA maxLength (IPv4, IPv6)
• ROA version
• ROA asID
• Certificate resource ASId