Add support for aggregates

[pdziepak]

One of the most significant limitations of codegen is its inability to handle aggregates. Addressing that is not trivial though since codegen will have to become aware of the target ABI. The initial implementation should support at least System V x86_64 ABI.

Another problem is integration with the C++ code. Since there is no static reflection (yet) the user will have to provide description of the types that are going to be passed to, or from code generated with codegen. Codegen should provide some high level helpers for that, something along the lines of codegen::tuple<...>.