CharBuf: constified template to_string; notes and doc fix.

CharBuf: made template to_string() const.
doc: notes on CharBuf::to_string() efficiency;
    corrected CharBuf::to_arithmetic() default note.
doc: fixed detail::string_traits multi-group inclusion.
More tidy; see full commit message.

Harder 80-column wrapping (especially on func decls).
Splayed class inheritance over multiple lines.
IO::write_arithmetic_array: made src const
VariableUtils: made convert_typed non-inline.