Release 0.6.0

Almost a complete rewrite. No functional change except it's faster.

- Ability to easily implement any quantum gate by simply giving an arbitrary unitary matrix in Gates.h
- Performance speedup using a single thread
- Some proper linear algebra primitives using template metaprogramming
- Abseil-cpp dependency for flat_hash_map
- Ability to compile the simulator with an set maximum number of qubits - in the future this will
be using templates as well to switch data structures based on the input quantum circuit

- Quantum state is stored as a sparse array implemented using a flat_hash_map from Abseil-cpp
- C++20 in place of C++14,
- Code style: camel case

- OpenMP and multithreading.
- SSE, AVX and other intrinsics.

include/qx/Version.h

@@ -1,4 +1,4 @@
 #ifndef QX_VERSION
-#define QX_VERSION "0.5.5"
+#define QX_VERSION "0.6.0"
 #define QX_RELEASE_YEAR "2023"
 #endif