Move Allocator patterns from std.heap to std.alloc

Because allocations are such a core pattern within Zig, the various
Allocators deserve a clear namespace of their own. Additionally a number
of the various allocators may be misleading or confusing within the heap
name namespace, e.g. `FixedBufferAllocator` where most of the examples
and uses use stack based memory, and not heap.

Additionally moving the various Allocators from `std.heap` into
`std.alloc` also improves the stdlib namespace. Where
`std.heap.ArenaAllocator` is now `std.alloc.Arena`.

CMakeLists.txt

@@ -392,6 +392,7 @@ set(ZIG_STAGE2_SOURCES
     lib/std/Thread/Pool.zig
     lib/std/Thread/ResetEvent.zig
     lib/std/Thread/WaitGroup.zig
+    lib/std/alloc/arena.zig
     lib/std/array_hash_map.zig
     lib/std/array_list.zig
     lib/std/ascii.zig
@@ -428,7 +429,6 @@ set(ZIG_STAGE2_SOURCES
     lib/std/hash/wyhash.zig
     lib/std/hash_map.zig
     lib/std/heap.zig
-    lib/std/heap/arena_allocator.zig
     lib/std/io.zig
     lib/std/io/Reader.zig
     lib/std/io/Writer.zig