* Move blockhash and fee_calculator in ThisInvokeContext instead of using a reference.
* Moves tx_wide_compute_cap into InvokeContext::push().
* Adds ThisInvokeContext::new_mock() constructor.
* Adds missing loader account in uses of MockInvokeContext.
* Use keyed_account_at_index() when accessing keyed_accounts.
* Makes sysvar interface consistent between ThisInvokeContext and MockInvokeContext,
in order to add InvokeContext::get_sysvars().
* Adds InvokeContext::set_blockhash() and InvokeContext ::set_fee_calculator().
* Adds new_mock_with_features.
* Makes ancestors optional in ThisInvokeContext.
* Adds prepare_mock_invoke_context() and mock_process_instruction().
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