replace solc crate by https://github.com/snd/rust_solc

[snd]

i've extracted the code that was used within sol-rs and the parity-bridge to shell out to solc into a crate at https://github.com/snd/rust_solc.

it enables compiling solidity files from rust with a single line of code, uses whichever of solc or solcjs is available, normalizes their outputs (renaming files), has tests and helpful error messages.

proposing to remove the solc folder from sol-rs and use https://github.com/snd/rust_solc instead.

[tomusdrw]

👍

[ddorgan]

[clabot: check]