lang: ast: re-factor condition group parser

this parser now only handles if-else statements, and uses the state init callback argument to receive any pre-existing inline expressions

lang/ast/expr/stmt.c

@@ -1,6 +1,5 @@
 #include "../node.h"
 #include "expr.h"
-#include "../cond.h"
 
 #include <blue/object/string.h>
 #include <ivy/lang/lex.h>
@@ -35,13 +34,12 @@ struct token_parse_result stmt_parse_if(
 		parser_pop_state(ctx, 0);
 	}
 
-	struct cond_group_parser_state *cond
-		= (struct cond_group_parser_state *)parser_push_state(
-			ctx, IVY_AST_COND_GROUP, 0);
-
 	/* if expr is NULL, this is an if-then-else-end statement,
 	 * otherwise, this is an expr-if-else-expr. */
-	cond->s_prev_node = expr;
+	struct cond_group_parser_state *cond
+		= (struct cond_group_parser_state *)parser_push_state(
+			ctx, IVY_AST_COND_GROUP, (uintptr_t)expr);
+
 	return PARSE_RESULT(IVY_OK, PARSE_REPEAT_TOKEN);
 }