I really don't know how to describe layout. I don't know if we should
call it a property of memory objects, specific values, types, or what.
I'm calling them objects here...

I gave all the links with a `p-` prefix that so that they can be
trivially copied and pasted into the other primitive types chapters.

This commit mostly adds links across the reference where things talked
about the boolean type. I also normalized the language so that it always
uses "boolean type". Other links were added as appropriate.

This commit also changes a few "x expressions" to "x operands" for if
and while expressions. The whole books needs this treatment, but while
I was here, I did it for these sections.

I also changed "`#[repr(C)]`" to "the C representation" in unions so
that I could actually link to them without breaking linkcheck.