feat(bin/linja): add linja

close #103, #108
feat(emacs/lean-settings): use linja for flycheck
chore(library/.gitignore): update
chore(library): add .project file
fix(frontends/let): let-expression pretty printer

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(build): invoke linja from lean build

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>