Add caching to flattened file compilation

[mikemcdonald]

The flattening on the current implementation seems wrong unless I'm missing something. It groups all solidity files in a directory and flattens them together, instead of getting a specific contract and flattening all of the dependencies with it.

This is a rough draft and still needs to be cleaned, but this PR:

• Moves the flattened folder outside of the contracts folder. Otherwise it gets messy when mixing compilers since hardhat doesn't have a great way to ignore folders
• Adds a cache for all of the flattened files so the compiler doesn't try to recompile every time when there are no updates

I still need to add a cache invalidator if the artifacts don't exist, in scenarios where compilation fails or is cancelled.

PS: the tests require a zk-solc binary in the path and matter-labs/compiler-llvm-context repo still needs to be open sourced in order to build it

[ly0va]

Hi @mikemcdonald, appreciate your input! This issue is already addressed in #4 which gets rid of the flattener altogether and makes use of HH's built-in dependency resolution and caching system.

[ly0va]

Closing this for now