Required for constraint solver islanding.
Replace linked lists with `island_{dof,efc}_{num,adr,ind}`, corresponding to the standard `{rownnz,rowadr,colind}` sparse matrix representation. By effectively defining two sparse matrix structures of dimensions `nisland x nv` and `nisland x nefc`, respectively, this representation is more conducive to reuse of existing sparse matrix utility functions, while being cache-friendlier by making sequential indices adjacent in memory.
PiperOrigin-RevId: 559704957
Change-Id: I919362cbef0d5fe5acc4aa2bb4ffb17eee8223a2
Yuval Tassa