Restructure Event to be cheap to clone and allow it to be rolled back

[jayz22]

What

Resolves #648 and #647.

Restructure the Event:

• Make it an internal structure that is cheap to clone. The buffer is using a MeteredVector such that metering is taken care of automatically.
• Converts to the external Event struct only by demand 1. when host is finished 2. when an error arises.

Include InternalEventsBuffer in the RollbackPointand roll backContractEvents (keeping DebugEvent`s) on contract failure.

Here is the sample output of a failed cross-contract call (contracts A->B and B failed):

=======Start of events=======
Debug event: rolled-back contract event: type I32(1), id Object(Bytes(#18)), topics Object(Vec(#13)), data Object(Vec(#17))
Debug event: rolled-back contract event: type I32(1), id Object(Bytes(#27)), topics Object(Vec(#23)), data Object(Vec(#26))
Debug event: TrapUnreachable
Debug event: U32(1) contract events rolled back. Rollback start pos = U32(1)
Debug event: contract call invocation resulted in error Status(VmError(TrapUnreachable))
Debug event: escalating error '' to VM trap
Debug event: VM trapped with host error
Debug event: U32(1) contract events rolled back. Rollback start pos = U32(0)
Debug event: contract call invocation resulted in error Status(VmError(TrapUnreachable))
========End of events========

Why

[TODO: Why this change is being made. Include any context required to understand the why.]

Known limitations

[TODO or N/A]