Changes to be committed:
* src/debruijn.ml    : rollback, old dbi were correct
* src/eval.ml        :
	- ctx is reinitialized between calls
	- new function debug_eval (equiv to old eval)
	- eval does not print call_trace and context if an error is raised
* tests/eval_test.ml : test ctx reinit
* tests/full_debug.ml: use debug_eval instead of eval

Changes to be committed:
* minor modifications