feat: gitignore echidna corpus

[gas1cent]

No description provided.

[simon-something]

hmm, not 100% certain we should do this. Reusing corpus is a huge benefit of Echidna, as every time you run it, it will start by replaying the previously "interesting" sequence of transaction. On the other hand, I get the issue of ever growing corpus dir in a repo...

We need a public good to compress/uncompress them (and only keep the latest coverage report)

[simon-something]

let's do it for now but let's try to find a better solution in the long run

[wei3erHase]

i support keeping the corpus out of the repo bc:

• would add a LOT of changes each time
• would mean all devs and processes share the same corpus (instead of each making their own)