class sinsp_filter
  ::compile(str)
call sinsp_filter::push_expression when entering a new nesting level (e.g. parens)
call sinsp_filter::parse_check to parse a single relational expression
     parse_check creates a sinsp_filter_check 'chk' of right type for field in this expression
this 'chk' holds the fieldname, operator, value, and also the boolean op that was "on the left" of the expression (or BO_NONE). Then it is added to the parent sinsp_filter_expression by calling sinsp_filter_expression::add_check




class sinsp_filter_expression : sinsp_filter_check
has a list of sinsp_filter_checks (m_checks)



class sinsp_filter_check // represents single relational expression


Summary: what we'll need to do:

- add an bool arg `lua_parsing` to sinsp::set_filter(const string& filter) (sinsp.cpp:1285)
  that bool (defaults false) is passed to the sinsp_filter constructor
- if true, sinsp_filter constructor will call lua_compile() instead of compile()
- add a new method sinsp_filter::lua_compile(const string& filter) (filter.cpp)
  this method calls up into lua with the string and some handle object that lua parser will use.

What lua parser can do with said handle:
- create a filter_expression
- create new sinsp_filter_check by calling g_filterlist.new_filter_check_from_fldname (filter.cpp:1483)
- set its comparison operator and previous bool operator (filter.cpp:1504)
- parse field name (filter.cpp:1506)
- parse value (filter.cpp:1610)