Use ptr_arg in Field::store_*/Field::val_*

This task could be splitted by two: one for store_* and one for val_*

We currently have following problem with Field class:

• It has methods to store and load values with respect to the data type conversion required:
  store_* and val_* (e.g. Field::val_float(), Field::store_str(),...)
• Those methods are virtual, and are overridden for certain ancestors to achieve a conversion to and from any fundamental type
• The problem is: they use Field::ptr internally instead of an explicit argument
• This leads to that in many places Field::ptr is changed to some value needed, then store_*/val_* ` is called, then it is changed back
  • in many places Field::move_field_offset() is used that. It could be potentially removed completely.

The task is:

• Fix all store_* or val_* functions to accept uchar *ptr_arg explicitly. I think it's better to make a new name
  • in particular, we'll soon have Field_varstring::val_str_from_ptr for exactly that purpose
• Eliminate hither-thither manipulations mentioned, across the code base, by locating Field::move_field_offset usages
• Add a method that uses Field::ptr back, but make it non-virtual
  • e.g.

    double Field::val_real() const

    { return val_real_from_ptr(ptr); }

    and so on