rust: properly package the C/C++ dependencies

[axic]

This reverts #584 mostly.

Now I feel dumb, but at least finally understood the reason: Cargo creates a tarball (with the .crate extension). It copies everything from the crate directory, including symlinks which it follows and includes fully.

The reason it works in evmc is that it can follow the single header symlink. The reason it did not work with symlinking to ../../ because that would be recursive for tar. Fixed it with symlinking the appropriate files/directories. I think this is the best we can do without moving the crate int the root and making it worse (by for example having two src directories or mixing Rust/C++ source code).

[codecov]

Codecov Report

Merging #585 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #585   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files          62       62           
  Lines        8990     8990           
=======================================
  Hits         8831     8831           
  Misses        159      159