Function cborToBigint

Convert CBOR to a bigint (any sign).

Matches Rust's TryFrom<CBOR> for BigInt.

Accepts:

• Major type 0 (unsigned integer)
• Major type 1 (negative integer)
• Tag 2 (positive bignum) with canonical byte string
• Tag 3 (negative bignum) with canonical byte string

Rejects:

• Floating-point values -> WrongType
• Non-canonical bignum encodings -> NonCanonicalNumeric