Plugin with evaluated call (#393)

* plugin trait

* impl of trait

* record and absolute path

* plugin example crate

* clippy error

* correcting cargo

* evaluated call for plugin

crates/nu-plugin/src/lib.rs

@@ -1,6 +1,8 @@
+pub mod evaluated_call;
 pub mod plugin;
 pub mod plugin_call;
 pub mod plugin_capnp;
 pub mod serializers;
 
+pub use evaluated_call::EvaluatedCall;
 pub use plugin::{serve_plugin, Plugin};