move get_columns from the table_viewer to a central location (#628)

* get_columns is working in the columns command

* the new location of the get_columns method is nu-protocol/src/column.rs

* reference the new location of the get_columns method

* move get_columns to nu-engine

crates/nu-engine/src/column.rs

@@ -0,0 +1,17 @@
+use nu_protocol::Value;
+
+pub fn get_columns(input: &[Value]) -> Vec<String> {
+    let mut columns = vec![];
+
+    for item in input {
+        if let Value::Record { cols, vals: _, .. } = item {
+            for col in cols {
+                if !columns.contains(col) {
+                    columns.push(col.to_string());
+                }
+            }
+        }
+    }
+
+    columns
+}

crates/nu-engine/src/lib.rs

@@ -1,9 +1,11 @@
 mod call_ext;
+pub mod column;
 mod documentation;
 mod env;
 mod eval;
 
 pub use call_ext::CallExt;
+pub use column::get_columns;
 pub use documentation::{generate_docs, get_brief_help, get_documentation, get_full_help};
 pub use env::*;
 pub use eval::{eval_block, eval_expression, eval_operator};