Improve tc::Group, tc::Cosets API.

This is a combination of many commits:

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final. add public/private for core classes.

wip - big cleanup cosets.

index<>, cosets<>, and group<> all defined

transparent <void> overrides that operate directly on indices, not names

inheritance hierarchy means one can drop or change generator names

still missing path support.

wip - named cosets

fix lang test

wip - named generators

Move Group docstring away from specialization.

Remove tc::Gen; use Group<void>

remove tc::Rel

add thousands separators for benchmark limits

add generators list to tc::group

remove pair_map

tc/CMakeLists.txt

@@ -1,13 +1,13 @@
 add_library(tc
     include/tc/core.hpp
-    include/tc/cosets.hpp
-    include/tc/group.hpp
     include/tc/groups.hpp
-    include/tc/pair_map.hpp
     include/tc/util.hpp
-    src/core.cpp
+
+    src/cosets.cpp
+    src/group.cpp
     src/groups.cpp
     src/lang.cpp
+    src/solve.cpp
     )
 target_link_libraries(tc peglib::peglib fmt::fmt)
 target_include_directories(tc PUBLIC include)