Rename vm to evm, move precompiles to evm module

[s1na]

See #455 for discussion

[coveralls]

Coverage remained the same at 95.126% when pulling a792437 on rename-evm into 74eabba on master.

[holgerd77]

Looks good.

[axic]

I'd actually argue that precompiles are part of the client and not the VM, but it may make sense this way if you plan to have wasm-based precompiles.

[s1na]

if you plan to have wasm-based precompiles

Yeah that was the main reasoning.

I'd actually argue that precompiles are part of the client and not the VM

I think I see your point and different precompiles (regardless of vm-type) would be distinguished by their address (i.e. there's no need to separate them based on vm-type). This was mostly a relocation of the precompile files in the directory structure, and doesn't affect the logic in any way.