feat(tvix/eval): introduce generators module

This module contains the request/response types for generators
requesting actions from the VM.

For most of these, an async helper function is added that will be used
inside of generator functions to make use of these requests/responses
instead of constructing them directly.

Change-Id: I1e085f88adaf784a34867957a0e82532d3a83d7c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8148
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/eval/src/value/mod.rs

@@ -32,7 +32,7 @@ pub use path::canon_path;
 pub use string::NixString;
 pub use thunk::Thunk;
 
-use self::thunk::ThunkSet;
+pub use self::thunk::{SharedThunkSet, ThunkSet};
 
 use lazy_static::lazy_static;