This adds strictly more information to the source files and reduces the
need for customized tooling to render the book.
(While this should not change the output of _rustbook_, it is very
useful when rendering the sources with external tools like Pandoc.)
This commit is an attempt to standardize the use of punctuation and
formatting in "The Rust Programming Language" as discussed in #19823.
- Convert bold text to italicized textcwhen referring to terminology.
- Convert single-quoted text to italicized or double-quoted text,
depending on context.
- Use double quotes only in the case of scare quotes or quotations.
