Fix snippets generation for sandboxed builds

[xzfc]

Two issues:

• For some reason we enable the generate-snippets feature for downstream users by default.
  Instead, this feature should only be used for our tests.
• build.rs writes files to source code directory.
  It can cause issues for sandboxed build systems (e.g., nix + crane: build.rs in v1.17.0 panics with PermissionDenied in Nix sandbox builds #268).
  Instead, generated files should go into $OUT_DIR. See https://doc.rust-lang.org/cargo/reference/environment-variables.html
  Also, that means we can drop autogenerated files from the repository.

This PR fixes both of them.

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Alternative solutions (not in this PR):

• Move snippet tests into a separate crate within the workspace. Do not ship this test crate to crates.io.
• If we want to put autogenerated files in tests/snippet_tests/, then we should do it by a separate script, not by build.rs.

[timvisee]

These are probably related: #235, #239

[generall]

In call we agreed to abandon "nice" version of the snippets, as we have snippets checker in documentation, and only keep generated version

[xzfc]

Superseded by #275.