Converts creation and deletion functions over to std::filesystem,
simplifying our file-handling code.
Notably with this, CopyDir will now function on Windows.
Migrates a remaining common file over to the Common namespace, making it
consistent with the rest of common files.
This also allows for high-traffic FS related code to alias the
filesystem function namespace as
namespace FS = Common::FS;
for more concise typing.
Previously, we were returning a value that was way too big, causing an integer overflow in Fractured Souls.
According to wwylele, the biggest oberserved save size for 3DS is 1MB, so this new value should leave plenty of room, even if games use a bigger size.
