refactor(library/init): move algebra to its own folder

library/init/algebra/default.lean

@@ -4,4 +4,4 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Leonardo de Moura
 -/
 prelude
-import init.group init.ordered_group init.ring init.ordered_ring
+import init.algebra.group init.algebra.ordered_group init.algebra.ring init.algebra.ordered_ring init.algebra.norm_num

library/init/algebra/norm_num.lean

@@ -4,7 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Robert Lewis and Leonardo de Moura
 -/
 prelude
-import init.ring
+import init.algebra.ring
 
 namespace norm_num
 universe variable u

library/init/algebra/ordered_group.lean

@@ -4,7 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Jeremy Avigad, Leonardo de Moura
 -/
 prelude
-import init.group
+import init.algebra.order init.algebra.group
 
 /- Make sure instances defined in this file have lower priority than the ones
    defined for concrete structures -/

library/init/algebra/ordered_ring.lean

@@ -4,7 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Jeremy Avigad, Leonardo de Moura
 -/
 prelude
-import init.ordered_group init.ring
+import init.algebra.ordered_group init.algebra.ring
 
 /- Make sure instances defined in this file have lower priority than the ones
    defined for concrete structures -/

library/init/algebra/ring.lean

@@ -4,7 +4,7 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Authors: Jeremy Avigad, Leonardo de Moura
 -/
 prelude
-import init.group
+import init.algebra.group
 
 /- Make sure instances defined in this file have lower priority than the ones
    defined for concrete structures -/

library/init/default.lean

@@ -12,5 +12,5 @@ import init.monad init.option init.state init.fin init.list init.char init.strin
 import init.monad_combinators init.set
 import init.timeit init.trace init.unsigned init.ordering init.list_classes init.coe
 import init.wf init.nat_div init.meta init.instances init.breakpoint
-import init.sigma_lex init.id_locked init.order init.algebra
+import init.sigma_lex init.id_locked init.algebra
 import init.nat_lemmas init.char_lemmas init.string_lemmas

library/init/nat.lean

@@ -5,7 +5,6 @@ Authors: Floris van Doorn, Leonardo de Moura
 -/
 prelude
 import init.relation init.num
-import init.order
 
 notation `ℕ` := nat