Handle state variables without next-function as inputs

[lonsing]

Issue refers to BTOR2 inputs and Pono master 5b6af14.

Excerpt from the BTOR2 paper (https://link.springer.com/content/pdf/10.1007%2F978-3-319-96145-3_32.pdf):

"A state variable without associated next function is treated as a primary input, i.e., it has the same behaviour as inputs defined via keyword input."

Pono properly handles inputs that are explicitly declared using the keyword input. Such inputs are added to the set inputvars_ of a transition system.

However, state variables declared using the keyword state and for which no next-function is given are added to the set statevars_ of a transition system although according to the BTOR2 standard they should appear in inputvars_.