fix(frontends/lean/elaborator): missing '\n' in error message

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/pp): pretty print 'let-expressions', closes #87

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean/pp): pretty print 'let-expressions', closes #87