Wrap keywords that were \terminal{} with \keyword{} instead

[ilazaric]

My impression is that keywords should be \keyword{<string>} usually, no surrounding command, except in one of grammar definitions, where they should be wrapped in a \terminal{...}

Didn't do this on a couple occurences in preprocessor.tex (if and else are in \terminal{} args):

\begin{bnf}\obeyspaces
\nontermdef{else-group}\br
    \terminal{\# else   } new-line \opt{group}
\end{bnf}

\begin{ncsimplebnf}\obeyspaces
\indextext{\idxcode{\#if}}%
\terminal{\# if     } constant-expression new-line \opt{group}\br
\indextext{\idxcode{\#elif}}%
\terminal{\# elif   } constant-expression new-line \opt{group}
\end{ncsimplebnf}

\begin{bnf}\obeyspaces
\nontermdef{if-group}\br
    \terminal{\# if     } constant-expression new-line \opt{group}\br
    \terminal{\# ifdef  } identifier new-line \opt{group}\br
    \terminal{\# ifndef } identifier new-line \opt{group}
\end{bnf}

I don't think these are quite the same meaning as keyword if

Also, there are many instances of keywords not being \keyword-ed in \tcode{} parts, this PR does not touch those

Additionally some minor cleanup

• expressions.tex: \keyword{U} -> \tcode{U}
• lex.tex: removed space between command and argument

[ilazaric]

Last commit fixes spacing in overloading.tex, bnf for operator (that I broke in previous commits to this PR)
I originally didn't realize the spacing was important
Rendered and it looks good: