Allow underscore in `Identifier`

common/src/identifier.rs

@@ -54,7 +54,8 @@ impl<T> Identifier<T> {
         let mut i = 0;
         while i < value.len() {
             if let Some(c) = char::from_u32(bytes[i] as u32) {
-                if c.is_ascii_lowercase() {
+                // TODO: Disallow leading underscore or multiple in a row?
+                if c.is_ascii_lowercase() || c == '_' {
                     i += 1;
                     continue;
                 }