Reuse namespaces

Currently, I believe the wrapper will break if you open, close, then reopen a namespace. For example:

```c++
namespace ns {
int a = 5;
}

namespace ns {
int b = 6;
}
```

Since the wrapper is not smart enough to realize that these are the same and hence it doesn't have to redefine the submodule `pybind11::module m_ns = ...`.

Aside from minor inconveniences of writing a `.i` file to remember to put all the stuff from a namespace at the same place, this causes issues using `combine_interface_headers` which just copy-pastes the contents together. Thus if you want to "add" to a namespace from a different interface header, it won't work.

Not sure whether or not the goal of having a nice pybind way to #include other `.i` files would be able to handle this, or if we just need some intelligent way of keeping track which namespaces have been handled. For example, a 2-pass approach: first go through all the namespaces and just define them and keep a `set()` of defined namespaces, then second pass do all the "real" wrapping.