Use a pure rust secp256k1 implementation

[luca992]

secp256k1 depends on secp256k1-sys a c wrapper. It has caused me build issues like having to update clang on my mac as well as being incompatible with rust-optimizer. Is there any reason to not use a pure rust implementation like k256? Plus it would be one less dependency since k256 is already a dependency of secret-cosmwasm-crypto

[KennySwayzee93]

Same problem here, would much appreciate a pure rust implementation