Add support for extended transaction format

[SupunS]

Issue To Be Solved

This issue is to track the implementation of FLIP: onflow/flips#41

Assuming the 'role block' syntax would be used, the following needs to be implemented:

• Support parsing role blocks/declarations in the parser
  • Add a new AST node to represent role declarations
  • Update the grammar
• Type check role declarations
  • Scoping rules would seamlessly work since it will be yet another block.
• Interpret role block
  • Ensure order: prepare, post and other similar blocks must be executed in the same order as the role definitions.
• Add documentation

TBD:

• Finalize the syntax and semantics for accessing fields across roles. i.e: accessing fields belonging to role A from outside of the role A.

[turbolent]

Support for transaction roles is done now. Remaining work is

• Maybe add support for pre and post conditions to role blocks. This has not been decided yet, so holding off for now
• Maybe add support for accessing the outer transaction from a role block. This has not been decided yet, so holding off for now
• Documenting the functionality, but blocked until the FLIP has been approved