Introduce wrapper eager_function

[eserie]

Introduce as_tensors, as_tensors_, as_raw_tensor, as_raw_tensors that rely in tree_flatten/tree_unflatten for more generic usages
JaxTensor is no longer registered as a pytree datastructure
Refactor JaxTensor._value_and_grad_fn

Fix #34

[codecov]

Codecov Report

Merging #41 (4583260) into master (4656839) will decrease coverage by 0.22%.
The diff coverage is 97.59%.

@@             Coverage Diff             @@
##            master      #41      +/-   ##
===========================================
- Coverage   100.00%   99.77%   -0.23%     
===========================================
  Files           16       16              
  Lines         1753     1802      +49     
===========================================
+ Hits          1753     1798      +45     
- Misses           0        4       +4     

Impacted Files	Coverage Δ
eagerpy/tensor/extensions.py	100.00% <ø> (ø)
eagerpy/tensor/base.py	99.05% <95.23%> (-0.95%)	⬇️
eagerpy/astensor.py	98.82% <97.95%> (-1.18%)	⬇️
eagerpy/__init__.py	100.00% <100.00%> (ø)
eagerpy/tensor/jax.py	100.00% <100.00%> (ø)
eagerpy/modules.py	94.11% <0.00%> (-5.89%)	⬇️

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