Using libsecp256k1 version 0.6.0

Update dependency libsecp256k1 to commit 0fa84f869d51e1b71113d81fcd518ebcee08709a (version 0.3.2)

BREAKS API:

For details see:
#36

Adds more documentation
Remove initializers on K1.ECDSA.NonRecoverable.Signature taking data and recoveryID, use compact: Compact init instead.
Remove recoverPublicKey method from K1.ECDSA.NonRecoverable.Signature, init a K1.ECDSA.Recoverable.Signature instead.
Add new method serialize:format on K1.ECDSA.Recoverable.Signature.Compact, also document that type.

Link to documentation at: https://swiftpackageindex.com/sajjon/k1/0.3.5/documentation/k1/k1/ecdsa

Various improvements to documentation with some API breakage

Keys are no longer typealiases, they are existentials (generated with gyb)

Major API break, see #16 and README for details.

Third release of 0.3.0 - using libsecp256k1 0.3.0.

Added more tests for ECDH and KDF.

Second release of 0.3.0 - using libsecp256k1 0.3.0.

Added more test for ECDH x963