feat: add dstructs to namespace

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Author: kgryte

lib/main.js

@@ -144,6 +144,15 @@ setReadOnly( stdlib, 'constants', require( '@stdlib/constants' ) );
 */
 setReadOnly( stdlib, 'datasets', require( '@stdlib/datasets' ) );
 
+/**
+* @name dstructs
+* @memberof stdlib
+* @readonly
+* @type {Function}
+* @see {@link module:@stdlib/dstructs}
+*/
+setReadOnly( stdlib, 'dstructs', require( '@stdlib/dstructs' ) );
+
 /**
 * @name error
 * @memberof stdlib