Nearly complete the callspec filtering feature

- still and outstanding bug with slices such as ts1.<csid>[:2]
  specified in the getattr/tabbed path
- bail with err msg in _tprint if no items are provided
replace more illegal chars, think about exc re-raising, shit is still badly broken
rework cs filtering using a more functional approach
Allow for params completion on FuncCollection

- use the callspec str id instead of calling str over values
- _children is superfluous
- getitem now works on params attrs as well
- for now leave out sub-node caching
Filter by params intersection not by the union

- solves issue with callspec-params based selection where
  any further selection would add tests by the union of callspec-params
  identifiers instead of filtering more tests out
- additional cleaning and simplifications
update todo list