Format indexed variables with code and maths properly

[tkoeppe]

We still have a number of "indexed" variables, like Ti or TiD, that should be formatted with the variable name in code and the index in maths, either \tcode{T}$_i$ or $\mathtt{T}_i$ or $\mcode{T}_i$ provided someone adds an \mcode macro, or $\text{\tcode{T}}_i$.

Please also look out for concrete indexes like $T_0$ etc.

[tkoeppe]

48efdfe contributed to this, but there's more work left to be done in [tuple.tuple].

[jensmaurer]

Waiting for #1142 to be applied to avoid merge conflicts.

[tkoeppe]

c214412 also contributed to this.

[jensmaurer]

@tkoeppe: Do you have a suggestion how to format TiD = decay_t<Ti> ? My guess would be T_i^D = decay_t<T_i> (with appropriate font macros), but maybe you've got another idea.

[tkoeppe]

@jensmaurer: How about TD_i = decay_t<T_i>? It's very unusual to have the index in the middle. Or maybe U_i, if that's available (I don't know which part you're looking at).