Fixed memory leak, made all prompts user-driven

* array_util
** Added function to prompt each form of the desired part of speech
** Part-of-speech prompts are now driven by the user pressing enter
* declension
** Changed prompt-generating code to be a call to prompt_forms
* verbs
** Changed prompt-generating code to be a call to prompt_forms
** Fixed memory leak caused by usage of strtok in concert with get_input