[Base] Add user-literals for several memory sizes

Rather than using `n * 1024 * 1024`, this adds a convenient `_MiB`/`_KiB` user-literal to the new `literals.h` header to concisely describe units of memory in a much more readable way. Any other useful literals can be added to this header. These literals exist in the `xe::literals` namespace so they are opt-in, similar to `std::chrono` literals, and require a `using namespace xe::literals` statement to utilize it within the current scope.

I've done a pass through the codebase to replace trivial instances of `1024 * 1024 * ...` expressions being used but avoided anything that added additional casting complexity from `size_t` to `uint32_t` and such to keep this commit concise.

src/xenia/base/arena.cc

@@ -60,7 +60,7 @@ void* Arena::Alloc(size_t size, size_t align) {
 
   if (active_chunk_) {
     if (active_chunk_->capacity - active_chunk_->offset <
-        size + get_padding() + 4096) {
+        size + get_padding() + 4_KiB) {
       Chunk* next = active_chunk_->next;
       if (!next) {
         assert_true(size + get_padding() < chunk_size_,