Various changes and improvements

* Don't put control characters to the command line buffer

* Avoid processing of unknown escape sequences

* Replace strcat()s with strcpy()s in stdio-less configuration

* Command history always ends with an empty line

* Move variable definitions out of for() statements

* Remove tkn_arr from microrl_t and use a stack to store this array

* Remove static variables and move them to microrl_t structure

* Move command line and history buffers out of microrl_t and place pointers there. Also, reorder some structure fields and use a memset() for initialization. These changes considerably reduce executable code size because field offsets are much smaller and they can be usually computed with a single instruction.