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

bump version to 0.12