v0.5.1

• Fix #169 (handling optional boolean, i32 and other primitive entity attributes incorrectly).
  • To unset primitive fields, entity.unset('fieldName') is now the way to go.
  • To explicitly check whether any field is set on an en entity, use entity.isSet('fieldName').
• Fix graphprotocol/graph-node#600 (data from different entities being mixed up due to non-deterministic use of previously used memory).