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

remove `ops::Index` impls for `Signature`

Various slice-handling ergonomic improvements

add `ThirtyTwoByteHash` hash trait which can be implemented for easier conversion of things to `Message`s

This makes more conceptual sense and does not add any new error paths, since even
valid `SecretKey`s were able to be invalid tweaks.

Update upstream libsecp; remove `without_caps` and all use of dummy contexts

export `serde`, `rand` and `libc`; update version to 0.11.5

Increase version to 0.11.4