Switch from free-functions to class structure

[coatless]

The PR implements logic to move toward C++ classes for each estimation model.

This simplifies passes and improves handling of each routine.

  expression          min   median `itr/sec` mem_alloc `gc/sec` n_itr  n_gc total_time 
  <bch:expr>     <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl> <int> <dbl>   <bch:tm> 
1 free_functions    13.6s    13.6s    0.0736   10.44MB   0.147      2     4      27.2s 
2 class_str         12.7s    13.6s    0.0739    4.46MB   0.0185     4     1      54.1s 

The class version has a lower memory consumption compared to the free function variant.