Added vString/subs-limit and vString/check-difference

subs-limit is a less error-prone variant of subs that uses a limit
instead of an end-value

check-difference compares two strings and returns a hash-map describing
differences.

Nice example of very simple piece of code that still has quite some
pitfalls:
  - string a shorter than b not detects
  - subs that fails when start and end not properly tested
Added vEdn/checked-edn to generate edn-strings.

Does a read-back to see whether the generated string was valid
and does detailed reporting to log-file in case of errors.

More detailed reporting possible in case of exception (is reporting in
inner loop reacheable?)
Added four join-functions that join multiple sets or lists

Each function can take an arbitrary number of xrel (lists or sets)
- inner-join
- left-outer
- right-outer
- full-outer