[util] Remove enum_bitfields

The enum_bitfields system never worked quite right, and always had edge cases where name
resolution for the SFINAE would fail. Move everything over to use util::bitset, which can
be constexpr and boils down to inline integer bitops in release mode.

Improved util::bitset itself, moving the array-backed base implementation into a new
util::sized_bitset, and making the single-inttype backed implementation the base case.
Also added a distinction between | or |= (which work with real bit values) and + or +=
(which work with bit indexes).

src/kernel/interrupts.cpp

@@ -126,8 +126,7 @@ isr_handler(cpu_state *regs)
             // The zero page is always invalid
             if (cr2 > mem::frame_size) {
                 bool user = cr2 < mem::kernel_offset;
-                vm_space::fault_type ft =
-                    static_cast<vm_space::fault_type>(regs->errorcode);
+                util::bitset8 ft = regs->errorcode;
 
                 vm_space &space = user
                     ? obj::process::current().space()