- Temp allocator now supports more than 2 in-flight stacks.

- Printing stacktrace uses its own temp allocator.
- `@pool` no longer takes an argument.
- `Allocator` interface removes `mark` and `reset`.
- DynamicArenaAllocator has changed init function.
- Added `BackedArenaAllocator` which is allocated to a fixed size, then allocates on the backing allocator and supports mark/reset.

lib/std/os/linux/linux.c3

@@ -204,7 +204,7 @@ fn void? backtrace_add_element(Allocator allocator, BacktraceList *list, void* a
 		return;
 	}
 
-	@pool(allocator)
+	@pool()
 	{
 		Linux_Dl_info info;
 		if (dladdr(addr, &info) == 0)
@@ -227,7 +227,7 @@ fn BacktraceList? symbolize_backtrace(Allocator allocator, void*[] backtrace)
 		}
 		list.free();
 	}
-	@pool(allocator)
+	@pool()
 	{
 		foreach (addr : backtrace)
 		{