tidy up lib/meta namespaces

[ichthyo]

currently they are within src/lib/meta/..., but the namespace is lumiera::typelist! Probably they should go below lib::, because they are part of the support lib, but don't belong to the external interface; additionally we should differentiate the namespaces, in order to make the resulting names more self-evident. E.g. "typelist::FunctionClosure" is misleading, while typelist::Append ist correct.

[ichthyo]

for now it's IMHO sufficient to have all of the metaprogramming lib in a single namespace lib::meta. I consider that part of the library roughly final by now. But it definitively doesn't belong into the interface namespace lumiera::typelist