restructure cmake project

example/CMakeLists.txt

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+add_executable(bench bench.cpp)
+target_link_libraries(bench PRIVATE tc)

example/bench.cpp

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+#include "solver.h"
+#include "groups.h"
+#include <chrono>
+#include <iostream>
+
+int main() {
+    tc::Group g = tc::group::T(2, 5000);
+
+    auto s = std::chrono::system_clock::now();
+    auto cosets = solve(g);
+    auto e = std::chrono::system_clock::now();
+
+    std::chrono::duration<double> diff = e - s;
+    int order = cosets.len;
+
+    std::cout << order << std::endl;
+    std::cout << diff.count() << std::endl;
+
+    return 0;
+}