extension FEATURE parsed tree in lysp_ext_instance

A pointer to the parsed tree is now available in the lysp_ext_instance
structure. Now it is not necessary to temporarily create a parsed tree
in the lys_compile_extension_instance function. The callback function
lyplg_ext_compile_clb had to lose the const qualifier for the
lysp_ext_instance parameter.
diff --git a/src/parser_yin.c b/src/parser_yin.c
index 889d2c3..3d408c4 100644
--- a/src/parser_yin.c
+++ b/src/parser_yin.c
@@ -3249,6 +3249,8 @@
         LY_CHECK_RET(lyxml_ctx_next(ctx->xmlctx));
     }
 
+    e->parsed = NULL;
+
     return LY_SUCCESS;
 }