mu-sexp: use std::vector instead of std::deque

Using deque gives compilation errors when compiling on
MacOS/clang (where it defaults to libc++ rather than gcc's libstdc++)

```
 #include <deque>
 struct Foo { std::deque<Foo> foos; };
 int  main() { Foo foo; }
```

So, let's use a vector instead; this is a drop-in replacement here, but
unfortunately in some future code...

lib/utils/mu-sexp.hh

@@ -24,7 +24,6 @@
 #include <iterator>
 #include <string>
 #include <vector>
-#include <deque>
 #include <type_traits>
 
 #include "utils/mu-utils.hh"
@@ -49,8 +48,9 @@ struct Sexp {
          */
         Sexp():type_{Type::Empty}{}
 
-        /// Underlying data type for list
+        // Underlying data type for list; we'd like to use std::dequeu here,
-        using Seq = std::deque<Sexp>;
+        // but that does not compile with libc++ (it does with libstdc++)
+        using Seq = std::vector<Sexp>;
 
         /**
          * Make a sexp out of an s-expression string.