rfc6979: use `crypto-bigint`

[tarcieri]

RFC6979 is defined in terms of integer operations, e.g.:

• bits2int
• mod
• shr
• int2octets

Our implementation previously avoided committing to any particular big integer library since dsa used num-bigint-dig and ecdsa used crypto-bigint. However, dsa has been migrated to crypto-bigint (#906), which means we can use crypto-bigint as the common big integer library for both without issue.