NifEncoder::encode should return a Result<NifTerm, Error> instead of just NifTerm

[hansihe]

As is, it is impossible to fail gracefully in NifEncoder::encode. This is a big design flaw, as things like allocation errors can happen when encoding a value.

When that is done, all existing encoders should be changed to fail gracefully instead of panicking.

[hansihe]

Thinking this through, it may not be as good of an idea as I initially thought. Changing this would increase how much code you would have to write to encode terms. It also seems like most errors (like allocation errors, write errors) are things you generally don't want to recover from. Closing for now.