Support for ScriptModule and methods

[dfalbel]

We need to make sure to:

• fix jit_trace strips names from lists of tensors #537
• Figure out how to share code between the ScriptModule R7 class and nn_Module so we only need to provide the basic types and all the rest is managed by the same code base.
• Assert returned modules are callable
• Assert modules returned by script$modules are also callable
• Make sure self$parameters_[[name]] correctly calls the register_parameter method.

Refactor the conversion from SEXP to torch::jit::Stack:

• Add lantern functions allowing to create IValues
• Create XPtrTorch subclasses for IValues and operators to convert from and to SEXPs
• Create Stack from IValues, instead of directly from R objects
• Refactor the callables creation for ScriptMethod and ScriptModule
• Add handling of vector of doubles with XPtrTorch subclasses.
• Add GenericDict handling types
• Add List handling type
• operator::SEXP() for XPtrTorchstring and support IValue <-> R character vector.
• Support NamedTuples IValue -> R
• Support named list R (with arbitrary types) -> GenericDict
• Support named list R (with arbitrary types) -> NamedTuple
• Think about how to clearly differentiate Tuple and List , NamedTuple and Dict - specially when they only contain tensors.
• Add a custom XPtrTorch type for the traced function.
• Fix windows builds
• Support for NULL type when tracing
• Improve jit documentation
• Add jit_scalar wrapper and convert length 1 numeric vectors to lists
• Support for optional types (probably can go in a different PR)