Rationalize folder structure

[awf]

As per discussion with @acl33 and @ryotatomioka

Where	Type	Language	What	Move to?
				_ _ setup _ _.py
/mlir/lib/Parser	lib	C++	Parser	src/lib/cpp/ksc
/src/python/ksc/	lib	Python	Parser/rewriter etc ksc.core.Expr ksc.torch.vmap	src/lib/python/ksc/core src/lib/python/ksc/torch
/rlo/src/rlo	lib	Python	DataConverter TorchModel expr_to_graph	src/lib/python/ksc/rlo src/lib/python/rlo-train
/mlir/ksc-mlir	exe	C++	.ks->.mlir	src/bin/ksc-mlir
/src/runtime	compilables	C++/ CUDA/ KS/ Futhark	includes etc for compilation of ks files	src/runtime
/src/ksc	exe	Haskell	.ks --AD--> .ks	src/bin/ksc
/src/onnx2ks	exe	Python		src/lib/python/onnx2ks src/bin/onnx2ks
/rlo/src	exe(s)	Python	user: .ks, model -> .ks train: scenario -> model check_rewrites, plotting etc	src/rlo/train_over_expressions/{toe1.py,toe2.py,main.py} src/rlo/evaluate_saved_model.py src/rlo/check_rewrites.py
/test	tests	Python		src/test