6.0.0 - Mary support

This release adds support for the Mary hardfork (adds multi-asset transactions)

BREAKING CHANGES:

• Many places that used to take a regular amount (BigNum) now take a Value which is a combination of BigNum + MultiAsset
• TransactionBody now contains a mint field
• TTL is now optional in transactions
• Ipv4, Ipv6, Url moved to separate network-specific structs with better error checking

New Features:

• New clamped_sub and compare functions added to BigNum
• Minimum ADA per UTXO increases when assets are added to a UTXO entry. To calculate the new minimum, you can use min_ada_required