Add commandline option to load bytecode for system contracts

[axic]

Proposed syntax:

• sys:<address>=<path>
• sys:sentinel=<path> (alias to 0x00...00e?)
• sys:evm2wasm=<path> (likewise)

For example, one would run testeth -t GeneralStateTests/stExample -- --testpath ~/tests --singlenet "Byzantium" --singletest "add11" --vm ~/libhera.so --evmc sys:evm2wasm=my_cool_version_of_evm2wasm.wasm

sys stands for system contract, while the address can be any ethereum address (hex encoded, all nibbles set, with the 0x prefix) and path is a relative filesystem path to load the binary from.

Additionally sentinel and evm2wasm are an aliases for their respective addresses.

The main benefit here is that we can load system contracts without adjusting the genesis and as a result makes testing way easier.

In the future we could think about migrating all these settings over to a configuration file.

[axic]

This feature should only change the code Hera will be executing when a call comes in to those addresses, but it cannot influence the client to create a new account and deploy code.

This second feature should be enabled by: ethereum/evmc#53

[jakelang]

@axic Figured as much. Should this be considered blocked on the evmc issue or should it be implemented the dirty way?

[axic]

It should only do case 1: replace the code hera has for the given account, but it is up to the client to actually route the call to hera.