- CT_LIT -> SHARED_THUNK
- CT_VAR only local vars, function names are converted to applications
- CT_APP dynamic/static. static case contains the correct descriptor pointer (to the slice)

+ faster way to check if thunk can be overwritten
+ check in the last two cases if arg is strict before evaluation

+ "eval" where cannot be avoided

+ introduce B stack

can be applied.

probably because most of the time the value removed immediately from the stack and set in a thunk
this way, this additional step can be saved

+ primitive functions set return value directly instead of returning it

+ execute a single VAR_FN when forced
+ refactor basic struct Desc definition to avoid cyclic includes