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_yang.c b/src/parser_yang.c
index b0609c7..c385f5c 100644
--- a/src/parser_yang.c
+++ b/src/parser_yang.c
@@ -917,6 +917,7 @@
 
     /* store the rest of information */
     e->format = LY_VALUE_SCHEMA;
+    e->parsed = NULL;
     e->prefix_data = ctx->parsed_mod;
     e->parent_stmt = insubstmt;
     e->parent_stmt_index = insubstmt_index;