Add AddressSpaceManager (#214)

This change brings in a new approach to managing address space.
It wraps the Pal with a power of two reservation system, that
guarantees all returned blocks are naturally aligned to their size. It
either lets the Pal perform aligned requests, or over allocates and
splits into power of two blocks.

README.md

@@ -194,16 +194,24 @@ pages, rather than zeroing them synchronously in this call
 
 ```c++
 template<bool committed>
-void* reserve(size_t size, size_t align);
-template<bool committed>
-void* reserve(size_t size) noexcept;
+void* reserve_aligned(size_t size) noexcept;
+std::pair<void*, size_t> reserve_at_least(size_t size) noexcept;
 ```
 Only one of these needs to be implemented, depending on whether the underlying
 system can provide strongly aligned memory regions.
-If the system guarantees only page alignment, implement the second and snmalloc
-will over-allocate and then trim the requested region.
+If the system guarantees only page alignment, implement the second. The Pal is 
+free to overallocate based on the platforms desire and snmalloc
+will find suitably aligned blocks inside the region.  `reserve_at_least` should 
+not commit memory as snmalloc will commit the range of memory it requires of what 
+is returned.
+
 If the system provides strong alignment, implement the first to return memory
-at the desired alignment.
+at the desired alignment. If providing the first, then the `Pal` should also 
+specify the minimum size block it can provide: 
+```
+static constexpr size_t minimum_alloc_size = ...;
+```
+
 
 Finally, you need to define a field to indicate the features that your PAL supports:
 ```c++