The mobile package provides a very narrow interface to ivy,
suitable for wrapping in a UI for mobile applications.
It is designed to work well with the gomobile tool by exposing
only primitive types. It's also handy for testing.
TODO: This package (and ivy itself) has global state, so only
one execution stream (Eval or Demo) can be active at a time.
Eval evaluates the input string and returns its output.
If execution caused errors, they will be returned concatenated
together in the error value returned.
TODO: Should it stop at first error?