Add helper functions for applying constructors.

[rupertlssmith]

Something like

construct : String -> List Expression -> Expression
fqConstruct : ModuleName -> String -> List Expression -> Expression

Constructors are currently built by applying the constructor as a function:

CG.apply [ CG.fun "SomeConstructor", arg1, arg2, arg3, ... ]

But the helper functions might make it more obvious how to apply constructors.