rephrase type irgen hook -> irext, to eliminate cases where it'd be very easy to accidentally cause infinite recursion of ir generation (e.g. slice(x) requiring evaluation of 'lhs.count', where lhs == slice(x)). also bring back slice hooks

src/eval/eval_ir.h

@@ -229,6 +229,7 @@ internal void e_expr_poison(E_Expr *expr);
 internal void e_expr_unpoison(E_Expr *expr);
 
 //- rjf: top-level irtree/type extraction
+E_TYPE_ACCESS_FUNCTION_DEF(default);
 internal E_IRTreeAndType e_irtree_and_type_from_expr(Arena *arena, E_Expr *expr);
 
 //- rjf: irtree -> linear ops/bytecode