Remove at_least

The Pal was providing policy for overallocating a block of memory to
achieve alignment make that part of the backend.
The backend should be responsible for layout policy.

src/test/func/fixed_region/fixed_region.cc

@@ -21,7 +21,8 @@ int main()
   // 28 is large enough to produce a nested allocator.
   // It is also large enough for the example to run in.
   // For 1MiB superslabs, SUPERSLAB_BITS + 4 is not big enough for the example.
-  auto [oe_base, size] = Pal::reserve_at_least(bits::one_at_bit(28));
+  auto size = bits::one_at_bit(28);
+  auto oe_base = Pal::reserve(size);
   Pal::notify_using<NoZero>(oe_base, size);
   auto oe_end = pointer_offset(oe_base, size);
   std::cout << "Allocated region " << oe_base << " - "

src/test/func/pagemap/pagemap.cc

@@ -68,7 +68,8 @@ void test_pagemap(bool bounded)
   // Initialise the pagemap
   if (bounded)
   {
-    auto [base, size] = Pal::reserve_at_least(bits::one_at_bit(30));
+    auto size = bits::one_at_bit(30);
+    auto base = Pal::reserve(size);
     Pal::notify_using<NoZero>(base, size);
     std::cout << "Fixed base: " << base << " (" << size << ") "
               << " end: " << pointer_offset(base, size) << std::endl;