configure: Move config files in config/ subdirectory
Move Json module into Std
IO: cleanup interface
Remove "cd" command.

Merlin should not rely on process cwd. Local directory is now derived
from buffername specified in reset command.
Cleanup a few warnings

Unused definitions / identifiers, few bugs, etc.
WIP Dot_merlin: make parsing pure
WIP Global_state: new module to manage of all global settings affecting merlin
WIP Clflags: take snapshot of current state
WIP Extension_utils: simplify interface
WIP State: rely on Global_state to list top-level modules
WIP Ocamlmerlin: rely on Global_state for initialization
DONE Command: rely on Global_state/Project to handle stateful config
configure: create src/config directory if needed
WIP introduce Cmi_cache for cache management
WIP merlin_typers: make typers use new cache management
DONE Update merlin to make use of new cache management
Add command to dump normal or extended environment
State: add parser validation to track changes in syntax (while toggling extensions)
typer: reset initial env when updating from root
dot_merlin: add CMI and CMT commands