Possible way to reduce size of state_t instance

[eao197]

There are two state_t members that have std::size_t type: state_t::m_nested_level and state_t::m_substate_count. On 64-bit platforms they are 64-bit values and occupy 16 bytes of memory.

But the maximum value of m_nested_level is limited by max_deep constant (that is 16 for the last decade and hardly be greater than 256 in the near future). And I'm in doubt that we need something larger that 16-bit value for m_substate_count (even if states for an agent are autogenerated it is hard to imagine a state machine with more than 64K substates for a state).

So if we change the type of m_nested_level to std::uint8_t and type of m_substate_count to std::uint16_t then we can save around of dozen bytes for every state_t instance.

[eao197]

A quick check shows that the size of state_t can be reduced from 160 to 152 bytes (kubuntu 22.04 + gcc-11). It seems that 8 bytes is too small gain to be merged into the master branch :(