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...
Seems there are problems compiling mu with XCode 11.6 (see build tests);
apparently because of libc++ being different from libstdc++.
clang++ builds works fine as long as we're using libstdc++.
