Refactored hyphen macros.

[henryso]

Fixes #1140.

No tests change.

Please review and merge if satisfactory.

[eroux]

Thanks, I think it's a good basis! But the main point for this feature is to allow the user to change the hyphen (font+char) according to the space the hyphen has... So I think it would be helpful to have a high level macro doing this (redefining \gre@hyphen in fact) a difficulty is to allow the user to change the hyphen automatically added in the Lua pass... Do you see what I mean? (I'm not sure it's 100% clear in my head)

[eroux]

In fact, in an ideal world, the second argument of \gre@hyphen would not be ignored in the case of hyphens added by Lua at the and of a line, it would be enddifference+width of (custos+space)... That requires the eolshift code to tell Lua which hyphen should be used though, but that could be done with an attribute... Wdyt?

[henryso]

I honestly don't know enough about this stuff to comment in any meaningful way. Let me know what I should do and I'll try to do it.

[eroux]

Hmmm... it's a bit blur in my mind too... maybe I can just continue the PR ? I have a few ideas on how to do what I think would be best...

[henryso]

Ok, I will pull this change into its own branch.