feat: allow passing in the source file as a parameter

[0xPhaze]

It would be nice to simply allow to pass in the path to a source file, e.g. hevm exec --contract Contract.sol:Contract which would recompile the source if outdated.
Currently I'm using the workaround hevm exec --code $(solc --bin-runtime src/Contract.sol | grep '^60\S*' -m 1), which could be made more robust (and currently isn't looking for a specific contract to compile).

[d-xo]

hm, yeah this could be a nice ux, I guess theres a few feature requests here:

1. making hevm exec aware of some project structure
2. being able to refer to artifacts from a solc output file by name
3. automatically calling the project build tool before running a contract

I think it would change the character of the exec subcommand a little, which up until now has been more of a one shot tool for ad hoc analysis, and the unit testing interface under dapp-test has been used for more structured work in an existing project, but it seems doable without breaking existing workflows. I would be open to merging if someone wants to do the work and make a PR.

[d-xo]

A simpler (and probably nicer) version would be to call solc on the given contract filename and parse the resulting output json directly. We could add a cli arg to allow users to choose a solc version, and perhaps respect any discovered remappings.txt files?