atomic_t<>: extend fetch_op to support cancellation

Use std::invoke inside atomic_op/fetch_op
Remove op_fetch because it's easily replaced
Add fetch_dec_sat algorithm (conditional decrement)

Utilities/cond.cpp

@@ -21,7 +21,7 @@ bool cond_variable::imp_wait(u32 _old, u64 _timeout) noexcept
 		verify(HERE), rc == WAIT_TIMEOUT;
 
 		// Retire
-		while (!m_value.fetch_op([](u32& value) { if (value) value--; }))
+		while (!m_value.fetch_dec_sat())
 		{
 			timeout.QuadPart = 0;