Split blocks and closures (#7075)

* Split closures and blocks

* Tests mostly working

* finish last fixes, passes all tests

* fmt

crates/nu-protocol/src/engine/capture_block.rs

@@ -3,7 +3,12 @@ use std::collections::HashMap;
 use crate::{BlockId, Value, VarId};
 
 #[derive(Clone, Debug)]
-pub struct CaptureBlock {
+pub struct Closure {
     pub block_id: BlockId,
     pub captures: HashMap<VarId, Value>,
 }
+
+#[derive(Clone, Debug)]
+pub struct Block {
+    pub block_id: BlockId,
+}