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/bench/named.cpp

@@ -0,0 +1,55 @@
+#include <string>
+#include <vector>
+
+#include <fmt/core.h>
+#include <fmt/ranges.h>
+
+#include <tc/core.hpp>
+
+template<typename G>
+void show(G const &g) {  // todo add formatter for groups, cosets.
+    fmt::print("  | ");
+    for (const auto &v: g.gens()) {
+        fmt::print("{} ", v);
+    }
+    fmt::print("\n");
+
+    for (const auto &u: g.gens()) {
+        fmt::print("{} | ", u);
+        for (const auto &v: g.gens()) {
+            fmt::print("{} ", g.get(u, v));
+        }
+        fmt::print("\n");
+    }
+}
+
+int main() {
+    tc::Group<char> group(4, {'r', 'g', 'b', 'y'});
+
+    group.set('r', 'g', 5);
+    group.set('g', 'b', 4);
+    group.set('b', 'y', 3);
+
+    show(group);
+
+    auto sub = group.sub({'r', 'g', 'y'});
+    show(sub);
+
+    auto res = sub.solve({});
+    fmt::print("res order: {}\n", res.order());
+
+    auto cos = sub.solve({'r', 'y'});
+
+    fmt::print("order: {}\n", cos.order());
+
+//    tc::Group<u_int8_t> group(4, {0, 1, 2, 3});
+//
+//    group.set(0, 1, 5);
+//    group.set(1, 2, 4);
+//    group.set(2, 3, 3);
+//
+//    show(group);
+//
+//    auto sub = group.sub({3, 2, 0, 1});
+//    show(sub);
+}