Remove all usage of `mem::uninitialized()` (#29)
* Remove all usage of `mem::uninitialized()` Backport from [upstream](https://github.com/rust-bitcoin/rust-secp256k1/pull/146) Closes #23 * Remove deprecated functions
* Remove all usage of `mem::uninitialized()` Backport from [upstream](https://github.com/rust-bitcoin/rust-secp256k1/pull/146) Closes #23 * Remove deprecated functions