implemented parser of a single line and top-level sections unified

- we store registered sections in the same format as subsections so that we
  can search/process them in a uniform way
- implemented find_item() that can parse both absolute and relative paths
- all parsers of basic types simplified to take care of only 1 value;
  arrays are handled by the caller
- implemented a part of the interpretter: it gets parsed line (attribute
  name, desired operations, and parameters) and loads the values into
  memory.  since declarations can be nested and they can span over multiple
  lines, the interpretter operates on a stack