Add human readable serialization and deserialization to Secret Key

Add no-std support

Fix `Rand` trait bounds.

Add as_mut_ptr to PublicKey

Relax cc dependency requirements.

- set allowed versions range for the `cc` dependency

Add raw context getter to Secp256k1 struct

Bumps rand to 0.6.X & associated updates

Changes include cargo-fix generated, Default impls

Test builds using cargo-web on Travis to ensure they dont break

bump version to 0.12.2

Stop checking context flags for secp256k1_ecdh as its not required

bump version to 0.12.1

Sync fuzztarget combine with tweak_add since real functions do that

Fix cargo warning output and wrong pointer width

Co-Authored-By: tomaka <[email protected]>

Some new fuzz tests I was writing ended up failing because two
nodes came up with different keys because adding public keys and
adding a tweak to a private key and multiplying by the generator
should get the same thing.

Remove dependency on libc

Fix cc dep as the rust ecosystem is terrible

Enable windows builds on travis