[DSLX] Support methods on structs

[cdleary]

As reflected in #526 an outstanding request has been to support methods on struct instances so folks can use the familiar "object oriented receiver" style function invocations -- without polymorphism the additional utility is limited but familiarity/convenience / some conciseness will certainly be increased, and it helps pave the path towards monomorphization, right now the DSL only supports parametric instantiation via constexpr values.