Fix compatibility with Rust 1.40

[ilammy]

Recently released Clippy 1.40 ships with this lint enabled by default. Since we run with "warnings as errors" policy, this lint causes build failures which is not nice.

error: needless `fn main` in doctest
  --> src/wrappers/themis/rust/libthemis-src/src/lib.rs:37:4
   |
37 | //! fn main() {
   |    ^^^^^^^^^^^^
   |
   = note: `-D clippy::needless-doctest-main` implied by `-D warnings`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_doctest_main

error: aborting due to previous error

I wouldn't say that explicit main() here is needless and some people share the same sentiment. However, since this is top-level module docs, we cannot disable this lint for this place specifically without turning it off in the entire module. Okay, Clippy, you win.

Checklist

• Change is covered by automated tests
• Benchmark results are attached (N/A)
• The coding guidelines are followed
• Public API has proper documentation
• Example projects and code samples are updated (no API changes)
• Changelog is updated if needed (minor change)

[vixentael]

technologies are weird

[ilammy]

Yay! More breaking changes in the toolchain with recent Rust 1.40 release. I'd just quote the commit:

Fix Cargo with "--features"

No idea what changed here, but for some reason Cargo does not allow to use "--features" with virtual crates now. However, it has no issues with it once we're in "themis" crate subdirectory and ask to document all crates in the workspace anyway.

Apparently, --features was not supposed to work with workspaces in the first place, but I have no clue why it worked before.

Hopefully, this unbreaks Rust build.

I've added it here instead of making a new pull request. I'll rename the PR into something more appropriate.