Store a reference to environment in session to tie its lifetime

[nbigaouette]

By storing a reference to the environment, a session cannot be built in a specific way that causes segmentation faults. This forces a let binding on the environment.

Unfortunately, the fix prevents the monomorphization trick employed for with_model_from_file() which had to be merged with with_model_from_file_monomorphized(). Since this should only be called once per program, I guess the cost is not that much.

Closes #42

[codecov-io]

Codecov Report

Merging #46 (7198ab1) into master (4a02bc0) will decrease coverage by 5.04%.
The diff coverage is 21.50%.

@@            Coverage Diff             @@
##           master      #46      +/-   ##
==========================================
- Coverage   19.18%   14.13%   -5.05%     
==========================================
  Files          17       18       +1     
  Lines         610      962     +352     
==========================================
+ Hits          117      136      +19     
- Misses        493      826     +333     

Impacted Files	Coverage Δ
...runtime-sys/src/generated/macos/x86_64/bindings.rs	0.00% <ø> (ø)
onnxruntime/src/session.rs	0.00% <0.00%> (ø)
onnxruntime/src/lib.rs	32.53% <41.02%> (+7.53%)	⬆️
onnxruntime/src/environment.rs	89.83% <50.00%> (+8.47%)	⬆️
onnxruntime/src/memory.rs	93.33% <100.00%> (ø)
onnxruntime/src/tensor/ort_tensor.rs	82.35% <100.00%> (ø)

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