fix(emacs/lean-mode): trigger eri-indent only at the beginning of line
feat(frontends/lean): check modification time of imported files

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(emacs/lean-company): respect lean syntax to extract findp prefix

close #207
feat(kernel/expr): add hash_bi function that takes binder information into account
fix(frontends/lean/info_manager): crash when accessing INFO in the end of the file
feat(frontends/lean/server): sort exact matches by size in FINDP
feat(library/definition_cache): store the hashcode of direct dependencies

Actually we store the has code of their types.
feat(library): add environment fingerprint
feat(*): use environment fingerprint to detect when the cache cannot be used because the configuration changed, closes #75

We are not taking into the account the options object yet.