Add ERC: Exponential Curves

[0xneves]

This ERC provides a function called expcurve that calculates the curve’s decay value at a given timestamp based on the initial timestamp, final timestamp, curvature, and curve direction (ascending or descending).

Discussion: https://ethereum-magicians.org/t/erc-xxxx-exponential-curves/20170

[eip-review-bot]

File ERCS/erc-7725.md

Requires 1 more reviewers from @axic, @g11tech, @SamWilsn, @xinbenlv

[github-actions]

The commit 6d2f5cd (as a parent of 3edcde2) contains errors.
Please inspect the Run Summary for details.

[SamWilsn]

Thanks for the contribution! I think this is a better fit for a library instead of a standard, and might be better as a part of OpenZeppelin (or similar library.)