Refactor LCD mod -

- pixel coordinates are always unsigned
- use `for ... in range(...)` instead of `while ...`
Enable zinc tests
Remove dummy test
Add tests to CharIO
Add tests to LCD
Starting to refactor dht22 demo
Hopefully better PT memory management with Rc
Do not emit mod pt if it is empty
Added lpc17xx integration pt test
Big chunk of work on PT

 - gdb now sets RUST_BACKTRACE
 - big refactoring of node::Node
   - submodes are now indexed by order of appearance
   - added materializer / mutator functions
   - added dependency tracking
 - fixes to parser to support all that
 - fixes to all pt code to support dependency-based tree walker
Various cleanig up; trying to fix dht22

This code doesn't work as well, it seems that rungs should be templated based on dht22 'a, S, P.
Fixed compilation error due to api mismatch
Tests are PITA, added hamcrest crate
dht22pt kind of works
All fields in Builder are now private
Task args are now extended with type params
Added zinc_task, that about to start working
PT now supports args with type params
Enabled app_dht22 on travis
Removed debug logging