Add Module::rootImports()
[Refactoring] TemplateInstance::semantic()
[Refactoring] ???
[Refactoring] scope.c
Clean up template instantiation and speculative calculation
Add TemplateInstance::tnext and construct 'instantiatino graph'

The `tinst` and `tnext` is the edge of the graph. `minst` will be used to
cache the result of `needsCodegen()`.
[Refactoring] fix indent in IsExp::semantic()
+todo, untested
+test