parsers CHANGE redesign API of all the parsers

Simplify work with different inputs by introducing ly_in handler similar
to the printers' ly_out. The input handler can be used repeatedly and
also some input data manipulation functions are added.

Add new lys_parse() as a generic function using ly_in input handler. The
current API (lys_parse_* functions) was rewritten to be a wrapper of
the generic lys_parse().

Next to the unit tests for the parsers functions, also the similar tests
for printers functions are added.
diff --git a/src/context.h b/src/context.h
index a1b771b..7dabc3e 100644
--- a/src/context.h
+++ b/src/context.h
@@ -18,6 +18,7 @@
 #include <stdint.h>
 
 #include "log.h"
+#include "parser_schema.h"
 #include "tree_schema.h"
 
 #ifdef __cplusplus