Move "chars" interface out of out.h and into mandoc.h. This doesn't

Move "chars" interface out of out.h and into mandoc.h.  This doesn't
change any code but for renaming functions and types to be consistent
with other mandoc.h stuff.  The reason for moving into libmandoc is that
the rendering of special characters is part of mandoc itself---not an
external part.  From mandoc(1)'s perspective, this changes nothing, but
for other utilities, it's important to have these part of libmandoc.
Note this isn't documented [yet] in mandoc.3 because there are some
parts I'd like to change around beforehand.