Improve MathML for \limits (#1897)
Improve MathML for \limits (#1897)

* Improve MathML for \limits

* Make code parallel

Khan/KaTeX,Khan/KaTeX,Khan/KaTeX,xymostech/KaTeX,xymostech/KaTeX,xymostech/KaTeX,xymostech/KaTeX,xymostech/KaTeX