Add secp256k1-sys with prefixed sources

This feature disables using the bundles sources and will link into
existing libsecp256k1 symbols.

Creating context from raw pointer

Fixes and tests for the ffi

Fix a safety problem and make the Context trait unimplementable

Updating libraries and small code improvements

1b0c5737 Remove trailing whitespace (Tony Jin)

Pull request description:

  Removes some trailing whitespace in the `macros.rs` file.

Top commit has no ACKs.

Tree-SHA512: cb0486f0c1660e56ce996e448315dc95799d9596657ea63fa3d1dc3fed83b4ed20ba42ea1e311a8a7da86982ee9bc5a61a6b874f0cfdd98bdda67ca762227c78

Adding usage examples

Add tests for no-std

Reintroduce rand-std feature

Add a feature to disable replacing C symbols with rust

Add a 'dont_replace_c_symbols' feature for not replacing C's symbols with rust. isn't compatible with no-std

Disable some useless lints