MSVC: handle symbol visibility

On MSVC, the exported symbols have to be marked both in the header and
in the implementation as dllexport, whereas on GCC the visibility=normal
attribute is only supposed to be used in the implementation. As a
result, we need two types of macros here, one for the public headers,
and the other for TUs with implementation. Also, the symbol name cannot
be generic ("API") because we only want to mark as dllexport those
symbols which are in a library that we're building *now*. Otherwise this
will break havoc in any libraries which also use the `ABI` macro *and*
also use libyang.
diff --git a/src/hash_table.c b/src/hash_table.c
index b07c3ad..d0c039a 100644
--- a/src/hash_table.c
+++ b/src/hash_table.c
@@ -156,7 +156,7 @@
     return 0;
 }
 
-API LY_ERR
+LIBYANG_API_DEF LY_ERR
 lydict_remove(const struct ly_ctx *ctx, const char *value)
 {
     LY_ERR ret = LY_SUCCESS;
@@ -262,7 +262,7 @@
     return ret;
 }
 
-API LY_ERR
+LIBYANG_API_DEF LY_ERR
 lydict_insert(const struct ly_ctx *ctx, const char *value, size_t len, const char **str_p)
 {
     LY_ERR result;
@@ -285,7 +285,7 @@
     return result;
 }
 
-API LY_ERR
+LIBYANG_API_DEF LY_ERR
 lydict_insert_zc(const struct ly_ctx *ctx, char *value, const char **str_p)
 {
     LY_ERR result;