Qdldl

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* Using the correct CMake variable for longs during compilation
* Using the correct CMake variable for longs during CI Testing

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* Export the version number in CMake and in preprocessor macros.
* Add CMake options to enable/disable building the shared/static libraries and demo executable.
* Add symbol visibility information to the public API and build a Windows import library.
* Rename all CMake options to have a `QDLDL_` prefix, e.g.
* `DFLOAT` -> `QDLDL_FLOAT`
* `DLONG` -> `QDLDL_LONG`
* `UNITTESTS` -> `QDLDL_UNITTESTS`

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* Reduce the amount of memory access in each iteration of the solver loops.

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* Fix to cmake when building `qdldl::qdldlstatic`.
* Fix to overflow issue when factoring very large matrices.

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* Julia implementation supports logical factorisation.
* Changed `QDLDL_bool` to be unsigned char everywhere, except for the Julia examples where it is now treated as `Uint8`.

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* Various cmake improvements.
* Added pure Julia implementation.