missing Unicode chars for some arrows

[NSoiffer]

MathJax output from mhchem builds up several arrows because there is no Unicode equivalent. This leads to poor speech without extra effort. E.g., for <=>, SRE generates

left harpoon with barb down minus under minus right harpoon with barb up

as opposed to "in equilibrium with". I suspect an actual font character would produce a superior visual rendering also, but that of course depends on the font implementation.

I saw missing characters for <=>, <<=>, and <=>> and asked some people on the Unicode Technical committee whether I missed spotting those characters given how many arrow chars they already have. The answer was that they are missing and encouraged me to submit a proposal to include any missing characters. Specifically one person said

...good idea looking for "macros" that build up symbols. If such use is widespread, with reasonably established usage, character encoding would seem appropriate.

@mhchem or @dpvc: if you can you list come up with a list of those chars, I'll work on getting a proposal together for them. If there are only three, it was suggested that they fill in the three remaining empty slots in "2B00 Miscellaneous Symbols and Arrows block".