[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/scheduler.cpp

@@ -339,5 +339,5 @@ scheduler::maybe_schedule(TCB *t)
         return;
 
     current_cpu().apic->send_ipi(
-        lapic::ipi::fixed, isr::ipiSchedule, cpu->id);
+        lapic::ipi_fixed, isr::ipiSchedule, cpu->id);
 }