The Most Efficient Known Addition Chains for Field Element & Scalar Inversion for the Most Popular & Most Unpopular Elliptic Curves

: I added the addition chain for secp256k1 field element inversion and corrected the authorship of the secp256k1 scalar inversion addition chain, based on feedback by Peter Dettman.
: I added the addition chain for Curve25519 scalar inversion.

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