decide what to do with underscores in section labels

[zygoloid]

Continuing from #1498, we have 81 section labels containing underscores. These look extremely ugly: the font we use to display section labels does not have a reasonable underscore character.

We should either:

• change the font used to render section labels (both in the body text and in xrefindex and xrefdelta) to have a less ugly underscore, or
• revise the section labels to avoid using underscores

Generally when we form a section label from an identifier, we change underscores into periods (eg, [unord.map.syn] not [unordered_map.syn]); fixing these section labels to do the same may be the most consistent option.

[jensmaurer]

Let's revise the section labels, avoiding the underscores.
@tkoeppe consistently talks us out of having more font creativity in the standard.

[tkoeppe]

That's right :-) I wouldn't be opposed to finding a solution that simply redefines this particular symbol, globally, to a somewhat more size-appropriate underscore, though. As long as we just have one kind of underscore... @godbyk: Any idea how to do that? It's the bold-roman body font that needs fixing.

[godbyk]

@tkoeppe Yes, we can redefine \textunderscore (which is used behind the scenes when you type _ in non-math mode) to draw our own underscore instead of using one from the font if we don't like the one from the font. By default, a 'fake' underscore is drawn if one isn't available in the font. It may be that the default fake one doesn't fit the design well and we should tweak it.

[timsong-cpp]

Speaking of ugly underscores, maybe we can do something about static_assert-declaration too.