[tlbuild] luaharfbuzz + tostring

[Simon Dales]

When testing the latest build of TeXLive: it would seem that harfbuzz
classes have invalid tostring()s

Lua reference says that tostring() is supposed to return a sensible
string for __any__ value.

Try running:
---------------
for k,v in pairs(luaharfbuzz) do
  print(k,v)
end
---------------
It should either:
* fail cos your particular lua hasn't got a table of that name.
* or list the keys and the tostrings of the values.

(other, longer/more robust scripts are available, but too much here)

Try it on "tex" and "string", ... .

//
The current version fails.

It could be a RaspberryPi thing. But the sources look like it won't be.

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I have done a hack that gets this test-script to work, but I have no
guarantee that it doesn't destabilise the library.

The hack removes the metatable.__tostring entries: "works". Either
iterate the zapping in lua or comment out the entry in the c sources.
Cave.

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Please test

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Simon