Add fn,def and value operations to slir

docs/examples/slir/hello_world.mlir

@@ -16,11 +16,13 @@ module @some.ns {
 
               // Def operator ----
               %baz = "serene.def"(%0){name = "baz"}: (i64) -> !serene.symbol
+              "serene.ret"(%baz){} : (!serene.symbol)
        },
        {
               %3 = serene.value 4 : i32
 
               // Def operator ----
               %baz1 = "serene.def"(%3){name = "baz"}: (i32) -> !serene.symbol
+              !serene.ret %baz1 : !serene.symbol
        }){name = "some-fn", return_type = i32} : () -> !serene.fn
 }

libserene/include/serene/slir/ops.td

@@ -122,17 +122,31 @@ def FnOp: Serene_Op<"fn", [
     let results = (outs SereneFn);
 }
 
-def ReturnOp: Serene_Op<"ret", [NoSideEffect, HasParent<"FnOp">,
-                                   ReturnLike, Terminator]> {
+def ReturnOp: Serene_Op<"ret", [
+  NoSideEffect, HasParent<"FnOp">,
+  ReturnLike, Terminator
+  //  MemRefsNormalizable
+]> {
 
     let summary = "This operation marks the return value of a function";
     let description = [{
-        ReturnOp
+        `ret` marks the return value of a function body.
+        Example:
+
+        ```mlir
+
+        ```
     }];
 
-    let arguments = (ins AnyType:$operand);
-    let assemblyFormat =
-          [{ attr-dict $operand  `:` type($operand) }];
+    let arguments = (ins Variadic<AnyType>:$operands);
+
+    let builders = [OpBuilder<(ins), [{
+        build($_builder, $_state, llvm::None);
+    }]>];
+
+    let assemblyFormat = "attr-dict ($operands^ `:` type($operands))?";
+    // TODO: Turn on the verifier for `ret`
+    // let hasVerifier = 1;
 }