[latexrefman] Add Arrows subsection

[Karl Berry]

I put the lasy definitions into common.texi (taking the math classes
from latexsym.dtx) and used the characters in the math symbols list.
Seemed to work ok. (I did not bother to commit new pdf/etc., though,
since I knew that would happen soon enough anyway.) Thanks for doing the
work.

It occurred to me that it might be nice to show the Unicode code point
(as textual information) even when the character is output, as in the input
  @BES{2135,\aleph} Aleph, transfinite cardinal (ordinary).
would produce
  \aleph  <alephchar> (U+2135) Aleph, trans ...

where the (U+xxxx) string is new. Wdyt? I'm not sure, but more
information seems better than less information? Overall, probably all
occurrences of @BES[U] need to be reviewed anyway for lacking control
sequences, random use when not needed in Texinfo, etc.

I also wondered about stating "must load latexsym" for the lasy symbols,
but failed to do it. (Didn't try it, but that's what latexsym.dtx says.)

I noticed there is a bug in (it seems) texinfo.tex resulting in
l.1466: Undefined cross reference `\accent-snt'.
and a few others (with texinfo.tex 2018-09-21.20). Looks like it happens
with commands that are already defined (\accent, \smash, \wlog, etc.).

Also some problem with "Declaration" and "Terminal input/output" (index
entries?).

I'll look into them sometime. Sigh. --thanks, karl.