Support functions with default arguments

Currently it is not possible to call the following function

```C++
int addOne(int val=0) { return val + 1; }
```
as
```Rust
add_one()
```
in Rust. The reason for this is that autocxx doesn't understand the default argument value.

The issue names no files, tests, or entry points. Start by locating how autocxx handles C++ default arguments while generating Rust bindings. Done means the shown C++ function can be called as `add_one()` from Rust, with coverage for that behavior.

Written by the indexing model from the issue text.