[WIP, don't merge] Non-type parameters in cxxt

[barche]

The following currently doesn't work:

cxx"""
           template<typename T, int R, int C=5>
           struct LikeEigen { T val = 5.0; };
"""
const LikeEigen{T,R} = cxxt"LikeEigen<$T,$(Val{R})>"

This example seems to work with the changes in this PR, but adding a calculation on the defaulted parameter (as is done in the Eigen lib) fails:

cxx"""
           template<typename T, int R, int C=5+R>
           struct LikeEigen2 { T val = 5.0; };
"""
const LikeEigen2{T,R} = cxxt"LikeEigen2<$T,$(Val{R})>"

This PR is mostly for reference, maybe it goes in the right direction but I don't really know what I'm doing here. It is also not urgent.