core: erradiction of null envs
core: final erradiction of null envs + tests
core: [build] only builds necessary pipelines for the requested result

the [fold] function used to traverse pipeline trees would visit all
descendants of a pipeline. There is now a guard function that may be
used to cancel the visit of a node and its descendants. This is used
by [build], so that if a dep is built, the function does not visit and
build its descendants. This could be considered as a performance bug.

The test is now more relevant to check dependencies are correctly
computed.
core: introduced new markings in cache

- in /cache: the actual cached file and the date of its calculation
- in /used: touched last time the pipeline was needed for something
- in /reqs: last time the pipeline was directly requested
core: history file for each target (requests, usages, builds)
core: datestamps in history
core: fixed bug introduced when changing fold
core: fixed previous commit
disabled syntax extension (breaks guizmin_mb_pipeline compilation)
labs: added bed_intersection_filter
Merge branch 'better-cache' of github.com:pveber/guizmin into better-cache
core: built log was incorrectly placed
console: cache selection by history
console: api to delete files in the cache