Enable use of global secp256k1 context (#1123)

* Enable use of global secp256k1 context

Context creation is expensive.

Use the preallocated context that ships with the library.

* Cargo fmt

* Set `secp256k1` crate as optional

* `global-context` feature depends on `std`
* Its usage scope was already limited to `std` environment

* ink-engine requires `secp256k1` feature