enhance Transact literal to support facts on transactions

[eoliphan]

Description

A key Datomic capability is the ability to reify transactions with arbitrary facts/metadata. E.g. the :user/id associated with the (transact). Continuing this example, perhaps a vase interceptor could extract the user identifier from a JWT, making it available for the subsequent transact.

Expected Behavior

Transact literal would allow user to specific attributes/values that should be associated with the transaction.