Fix interaction between styles and sizes. (#719)

* Fix interaction between styles and sizes by implementing styles as sizes.

Rather than having both `textstyle` CSS classes and `size5` CSS classes
affect the font size (and step on each other), implement sizes more the
way TeX does: a command like `\displaystyle` changes the current size.

This is actually a simplification, since now only `size` affects the size.
Simplifies CSS and computation. Many screenshotter tests change; they
change to be more like TeX. For instance, `\sqrt` fixes some
discrepancies in size treatment.

Also:

Remove the `Options.withX()` methods in favor of `.havingX()`, which
might return the same `options`.

Remove `Style.cls()` and `Style.reset()`.

Remove `Options.reset()`. You should never modify an `Options`; they
should change only by the `havingX()` methods.

* Implement TeX sizing for scriptsize/scriptscriptsize.

At every size level. Also make the sizes match TeX to the last decimal.

* Review comments.

src/buildMathML.js

@@ -480,7 +480,8 @@ groupTypes.styling = function(group, options) {
 };
 
 groupTypes.sizing = function(group, options) {
-    const inner = buildExpression(group.value.value, options);
+    const newOptions = options.havingSize(group.value.size);
+    const inner = buildExpression(group.value.value, newOptions);
 
     const node = new mathMLTree.MathNode("mstyle", inner);
 
@@ -489,8 +490,7 @@ groupTypes.sizing = function(group, options) {
     // in, so we can't reset the size to normal before changing it.  Now
     // that we're passing an options parameter we should be able to fix
     // this.
-    node.setAttribute(
-        "mathsize", buildCommon.sizingMultiplier[group.value.size] + "em");
+    node.setAttribute("mathsize", newOptions.sizeMultiplier + "em");
 
     return node;
 };