Begin converting to new built-in API

src/built_in_functions/assert.rs

@@ -0,0 +1,19 @@
+use crate::{BuiltInFunction, Error, Result, Value};
+
+pub struct Assert;
+
+impl BuiltInFunction for Assert {
+    fn name(&self) -> &'static str {
+        "assert"
+    }
+
+    fn run(&self, arguments: &[Value]) -> Result<Value> {
+        for argument in arguments {
+            if !argument.as_boolean()? {
+                return Err(Error::AssertFailed);
+            }
+        }
+
+        Ok(Value::Empty)
+    }
+}