Replace Eigen block operations with small GEMM and GEMV loops.

1. Add Matrix-Matrix and Matrix-Vector multiply functions.
2. Replace Eigen usage in SchurEliminator with these custom
   matrix operations.
3. Save on some memory allocations in ChunkOuterProduct.
4. Replace LDLT with LLT.

As a result on problem-16-22106-pre.txt, the linear solver time
goes down from 1.2s to 0.64s.

Change-Id: I2daa667960e0a1e8834489965a30be31f37fd87f