Support the onnx string type in output tensors

This approach allocates owned Strings for each element, which works, but stresses the allocator, and incurs unnecessary copying.

Part of the complication stems from the limitation that in Rust, a field can't be a reference to another field in the same struct. This means that having a Vec<u8> of copied data, referred to by a Vec<&str>, which is then referred to by an ArrayView, requires a sequence of 3 structs to express. Building a Vec<String> gets rid of the references, but also loses the efficiency of 1 allocation with strs pointing into it.

onnxruntime/tests/integration_tests.rs

@@ -19,6 +19,7 @@ mod download {
         tensor::{DynOrtTensor, OrtOwnedTensor},
         GraphOptimizationLevel, LoggingLevel,
     };
+    use onnxruntime::tensor::ndarray_tensor::NdArrayTensor;
 
     #[test]
     fn squeezenet_mushroom() {