Implement in, out and inout parameter passing modes

The `in T` type optimises type `T` for passing as an input parameter.
The `out T` type optimises type `T` for passing as an output parameter.
The `inout T` type (or `io T` optimises type `T` for passing as an input/output parameter.

This relies on [binding](http://c3d.github.io/xl/#binding) binding rules, and selecting passing by copy or by reference depending on what is more efficient.