Replace DenseSlice 2D array with flat vector for full contiguity

[mitjarislakki]

Currently allocates a vector of vectors, flat should improve cache coherence

liblsqecc/src/patches/dense_slice.cpp

Lines 108 to 113 in 3b80804

	DenseSlice::DenseSlice(const Layout& layout)
	: cells(std::vector<RowStore>(
	layout.furthest_cell().row+1,
	RowStore(layout.furthest_cell().col+1, std::nullopt))),
	layout(std::cref(layout))
	{