Fix up eval params and refactor

crates/nu-engine/src/lib.rs

@@ -1,3 +1,7 @@
 mod eval;
+mod state;
+mod value;
 
-pub use eval::{eval_block, eval_expression, eval_operator, ShellError, Stack, State, Value};
+pub use eval::{eval_block, eval_expression, eval_operator, ShellError};
+pub use state::{Stack, State};
+pub use value::Value;