Switch to FixedArray implementation from abseil.

This PR changes the implementation of the current fixed array to the
abseil one, which has proper allocator support.
Some minor changes are made to make the fixed array implementation
self-contained (no dependent to abseil):
- No address sanitizer support (red zones, etc.)
- Remove of noexecpt specified for copy and move constructor.
- Remove of 'at' function as Ceres does not use exceptions.
- Use std::tuple instead of absl::CompressedTuple as it uses the  abseil
  utility header which includes a whole bunch of other headers.

Change-Id: I43445b42c37f944509b5353a587d0efce74cbccf

include/ceres/autodiff_first_order_function.h

@@ -86,8 +86,8 @@ namespace ceres {
 // first, and are passed as const pointers to arrays of T. The
 // output is the last parameter.
 //
-// Then given this class definition, the auto differentiated FirstOrderFunction for
-// it can be constructed as follows.
+// Then given this class definition, the auto differentiated FirstOrderFunction
+// for it can be constructed as follows.
 //
 //    FirstOrderFunction* function =
 //      new AutoDiffFirstOrderFunction<QuadraticCostFunctor, 4>(
@@ -132,7 +132,7 @@ class AutoDiffFirstOrderFunction : public FirstOrderFunction {
     output.a = kImpossibleValue;
     output.v.setConstant(kImpossibleValue);
 
-    if (!(*functor_)(x.get(), &output)) {
+    if (!(*functor_)(x.data(), &output)) {
       return false;
     }