Improvements to SymbolTable::gates function (#217)

* Filter U gate from output of SymbolTable::gates

The Qiskit importer otherwise needs to filter this gate out.
Other consumers probably would do so as well. If it is useful in
antother context, we can add an option to include the U gate

Review comment: Qiskit/qiskit#12087 (comment)

* Use impl trait for SymbolTable::gate return type
So we return an iterator, not a `Vec`.

crates/oq3_semantics/src/symbols.rs

@@ -249,20 +249,22 @@ impl SymbolTable {
             .collect::<Vec<_>>()
     }
 
-    /// Return a Vec of information about all gate declarations. Each element
+    /// Return an iterator giving information about all gate declarations. Each element
     /// is a tuple of (gate name, symbol id, num classical params, num quantum params).
-    pub fn gates(&self) -> Vec<(&str, SymbolId, usize, usize)> {
-        self.all_symbols
-            .iter()
-            .enumerate()
-            .filter_map(|(n, sym)| {
-                if let Type::Gate(num_cl, num_qu) = &sym.symbol_type() {
-                    Some((sym.name(), SymbolId(n), *num_cl, *num_qu))
-                } else {
+    /// `gphase` is not included here. It is treated specially.
+    /// `U` is also filtered out, as it is builtin.
+    pub fn gates(&self) -> impl Iterator<Item = (&str, SymbolId, usize, usize)> {
+        self.all_symbols.iter().enumerate().filter_map(|(n, sym)| {
+            if let Type::Gate(num_cl, num_qu) = &sym.symbol_type() {
+                if sym.name() == "U" {
                     None
+                } else {
+                    Some((sym.name(), SymbolId(n), *num_cl, *num_qu))
                 }
-            })
-            .collect()
+            } else {
+                None
+            }
+        })
     }
 
     /// Return a list of hardware qubits referenced in the program as a