Fix lifetimes for NifEncoder and NifDecoder

[hansihe]

At the moment NifEncoder can't encode borrowed values that are not statically lived. NifDecoder should also be able to return values borrowed directly from the BEAM (this is safe as long as the borrow is of the same lifetime as the NifEnv object).

I tried a few things to make this work, but I am a bit unsure how lifetimes work in conjunction with traits. More experimentation is needed, I should get back to this later.

[hansihe]

One consequence of fixing this issue is that it would allow NifEncoder to encode &strs that are not statically lived. Example: In ::map::make_ex_struct