Port str to-decimal to into decimal command. (#408)

* Port str to-decimal to into decimal command. Add also a Value::test_float function for tests only

* Add support for handling integers into decimals and fix issues with error span

crates/nu-protocol/src/value/mod.rs

@@ -631,6 +631,14 @@ impl Value {
             span: Span::unknown(),
         }
     }
+
+    // Only use these for test data. Span::unknown() should not be used in user data
+    pub fn test_float(val: f64) -> Value {
+        Value::Float {
+            val,
+            span: Span::unknown(),
+        }
+    }
 }
 
 impl Default for Value {