function getParamAtIndex(signature, index) {
      return index < signature.params.length
          ? signature.params[index]
          : hasRestParam(signature.params)
              ? last(signature.params)
              : null
    }
Find the parameter at a specific index of a signature.
Handles rest parameters.
@param {Signature} signature
@param {number} index
@return {Param | null} Returns the matching parameter when found,
                       null otherwise.