Documentation strings for routines in 'tcalc.c'.
Column numbers now start from 1 instead of 0.
Added data structures for expression parsing.

New files added: 'queue.c', 'queue.h', 'stack.c' and 'stack.h'
containing implementation of queue and stack data structures used for
expression parsing (previously they were linked as libraries).

Modified the Makefile to compile and link the new files.

Added a header doc to 'tcalc.c'.
Add documentation strings.

Add documentation strings in 'expr_parser.c' and 'expr_parser.h'.
Slight code rearrangement in 'tcalc.c'