Revert "remove PublicKey::combine"

This reverts commit 17450e91.

Update version to 0.11

switch to 64-bit compilation; add flag for 32-bit

remove PublicKey::combine

add Debug impl to context object

add Display impl for the key types which outputs their hex representation

improve documentation

Since making PublicKey::serialize return a fixed-size array, this no longer
requires any allocation, so there's no reason not to have it.

Fix typo

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