Maximum number of transactions allowed in a package is 25, equal to the
default mempool descendant limit: if a package has more transactions
than this, either it would fail default mempool descendant limit or the
transactions don't all have a dependency relationship (but then they
shouldn't be in a package together). Same rationale for 101KvB virtual
size package limit.
Note that these policies are only used in test accepts so far.
Define the Package type as an alias for a vector of transactions for now.
Add PackageValidationResult, similar to TxValidationResult and
BlockValidationResult for package-wide errors that cannot be reported
within a single transaction result, such as having too many
transactions in the package. We can update the concept of
what a package is and have different logic for packages vs lists of
transactions in the future, e.g. for package relay.
