fixes of Set based on BB1.6. Refs: #95.
- no state machine involved, changes are based on BB1.6 version
- s.start fixed: set to the position of the opening "{"
- s.type set to invalid in case of end-of-text within a set
- Skip before "}" removed because it is redundant

Signed-off-by: Josef Templ <josef.templ@gmail.com>