Work towards: #5

Description

Also adds a decoder for evm.TransactionExecutedEvent payloads and storing these to the in-memory storage.

For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels