Update a couple dependencies

bump version to 0.10.0; introduce CHANGELOG.md

add feature-gated serde 1.0 support for Signature, SecretKey, PublicKey

remove `PublicKey::new()` and `PublicKey::is_valid()`

Minor fixes

build.rs: change build flags to eliminate compiler warnings

Use latest version at docs.rs

Update secp256k1 dependency

It was removed from bitcoin-core/libsecp256k1 at e06e878fd7dcc99825025fa99aedb86bc7d5c29f

Improve API ergonomics

This introduces the actual breaking API change.

Remove ContextFlag enum
Remove InvalidContext error-enum variant
Remove unused imports

The new API allows us to remove a bunch of tests which are now checked
by the compiler.

Add type parameter to Secp256k1
Add PhantomData for C
Separate into structs and traits
Move constructors to own impl blocks

bump version to 0.9.2

impl fuzztarget secp256k1_ecdsa_signature_serialize_der

 Implement Ord for arrays

ignore intellij files

Fix fuzztarget ECDH to be symmetric

Fix for older rustcs, removing serde/rustc-serialize and a missing include in fuzztarget