Output <code> instead of <tt> for inline literals in rst files

When encountering inline literals in rst, `<tt>` is output by rest2html, because that's what docutils generally does. The renderer for every other format outputs `<code>` for inline literals. Not accounting for `<tt>` results in emoji being inserted into rst inline literals, an issue which I also submitted a pull request for here: @jch/html-pipeline#147.