Dependencies: Add lru-cache library

Adds a generic C++ based caching library to support material external
modules. This is a template based library which means there is nothing
to build or link.

The only modification is to the CMakeLists.txt file to enable
integration with FreeCAD,

Original source: https://github.com/goldsborough/lru-cache

src/3rdParty/lru-cache/examples/statistics.cpp

@@ -0,0 +1,76 @@
+/// The MIT License (MIT)
+/// Copyright (c) 2016 Peter Goldsborough
+///
+/// Permission is hereby granted, free of charge, to any person obtaining a copy
+/// of this software and associated documentation files (the "Software"), to
+/// deal in the Software without restriction, including without limitation the
+/// rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+/// sell copies of the Software, and to permit persons to whom the Software is
+/// furnished to do so, subject to the following conditions:
+///
+/// The above copyright notice and this permission notice shall be included in
+/// all copies or substantial portions of the Software.
+///
+/// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+/// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+/// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+/// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+/// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+/// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+/// IN THE SOFTWARE.
+
+#include <iostream>
+
+#include "lru/lru.hpp"
+
+using Cache = LRU::Cache<std::uint64_t, std::uint64_t>;
+
+std::uint64_t fibonacci(std::uint64_t n, Cache& cache) {
+  if (n < 2) return 1;
+
+  // We std::uint64_ternally keep track of the last accessed key, meaning a
+  // `contains(key)` + `lookup(key)` sequence will involve only a single hash
+  // table lookup.
+  if (cache.contains(n)) return cache[n];
+
+  auto value = fibonacci(n - 1, cache) + fibonacci(n - 2, cache);
+
+  // Caches are 100% move-aware and we have implemented
+  // `unordered_map` style emplacement and insertion.
+  cache.emplace(n, value);
+
+  return value;
+}
+
+std::uint64_t fibonacci(std::uint64_t n) {
+  // Use a capacity of 100 (after 100 insertions, the next insertion will evict
+  // the least-recently inserted element). The default capacity is 128. Note
+  // that for fibonacci, a capacity of 2 is sufficient (and ideal).
+  Cache cache(100);
+  cache.monitor(2, 3, 4, 5);
+  auto value = fibonacci(n, cache);
+
+  for (auto i : {2, 3, 4, 5}) {
+    auto stats = cache.stats().stats_for(i);
+    // clang-format off
+    std::cout << "Statistics for " << i << ": "
+              << stats.hits << " hit(s), "
+              << stats.misses << " miss(es)."
+              << std::endl;
+  }
+
+  // You'll notice we'll always have n - 1 misses, for each time we access
+  // one of the numbers in [0, n] for the first time.
+  std::cout << "Overall: "
+            << cache.stats().total_hits() << " hit(s), "
+            << cache.stats().total_misses() << " miss(es)."
+            << std::endl;
+  // clang-format on
+
+  return value;
+}
+
+auto main() -> int {
+  // The last number that fits into a 64 bit unsigned number
+  std::cout << fibonacci(92) << std::endl;
+}