enable PEs with multiple reg/mem slave interfaces

Update `libtapasco` dependencies

because `structopt`'s functionality is now completely integrated in `clap`.

The generated context selectors now have the `Snafu` suffix.

The old interface was deprecated in version 3.0.0.

The methods used before were deprecated in the new version of the `config` crate.

The `get_str` method on `Config` struct was renamed to `get_string`.

Aside, the minimum Rust compiler version supported is now 1.49.0.

From `snafu-upgrade-assistant`:
In SNAFU 0.7, generated context selectors now have the Snafu suffix to help de-mystify the generated code.

From SNAFU's Changelog:
- Generated context selectors now have the suffix Snafu. This is a breaking change.
- Minimum supported version of Rust is now 1.34.

Remove the `volatile` crate in favor of the
`std::ptr::{read,write}_volatile` functions.

Also add `Cargo.lock` for `libtapasco_tests`.