The goal here is to override the CALL opcode so we can replace calls to
ovmSLOAD to native SLOAD operations. To start we've overridden the
simpler MSTORE opcode and print "MSTORE Overriden" to verify the opcode has been
overriden.

The goal is to override he CALL opcode and replace calls to "ovmStore"
with native SSTORE calls. This first step runs a call operation and
verifies that the correct arguments were passed in.

The next step is to actually hook up these opcodes to native SLOAD and
SSTORE

Th remaining bytes are parameter data

..instead of the "success code"

* Use an environment variable to set EXECUTION_MANAGER_ADDRESS

* Check that `len(args)` is greater than 4

When calling the shimmed SSTORE and SLOAD we were storing to and loading
from the ExecutionMananger's storage space. Instead we now call from the
context of the `caller` of the contract. This gives each contract its
own storage space so state isn't clobbered between contracts.