[upstream_utils] Upgrade to Sleipnir 0.3.2 (#8323)

Also includes a C++ benchmark, which has a Java counterpart in #8236.

wpimath/src/main/native/cpp/controller/ArmFeedforward.cpp

@@ -19,7 +19,7 @@ wpi::units::volt_t ArmFeedforward::Calculate(
     wpi::units::unit_t<Angle> currentAngle,
     wpi::units::unit_t<Velocity> currentVelocity,
     wpi::units::unit_t<Velocity> nextVelocity) const {
-  using VarMat = slp::VariableMatrix;
+  using VarMat = slp::VariableMatrix<double>;
 
   // Small kₐ values make the solver ill-conditioned
   if (kA < wpi::units::unit_t<ka_unit>{1e-1}) {
@@ -40,7 +40,7 @@ wpi::units::volt_t ArmFeedforward::Calculate(
 
   Vectord<2> r_k{currentAngle.value(), currentVelocity.value()};
 
-  slp::Variable u_k;
+  slp::Variable<double> u_k;
 
   // Initial guess
   auto acceleration = (nextVelocity - currentVelocity) / m_dt;

wpimath/src/main/native/cpp/geometry/Ellipse2d.cpp

@@ -20,7 +20,7 @@ Translation2d Ellipse2d::Nearest(const Translation2d& point) const {
 
   // Find nearest point
   {
-    slp::Problem problem;
+    slp::Problem<double> problem;
 
     // Point on ellipse
     auto x = problem.decision_variable();