Fix emcee problem with older versions of slang

Slang v2.3.1 and older did not allow keywords as unquoted field namse. The
implementation uses 'continue' as a qualifier and passed it arround like a
normal struct field. This caused problems with said older versions.
Changed to use get_struct_field/set_struct_field. Still the problem when
used as the qualifier to pass to emcee_hammer, but can be quoted like

emcee_hamme(; "continue");