Lua 5.5: for variables are const

[Zash]

In Lua 5.5 beta, for loop variables are const, resulting in errors such as this:

$ lua5.5 -l pl.tablex
lua5.5: error loading module 'pl.tablex' from file '/usr/share/lua/5.5/pl/tablex.lua':
	/usr/share/lua/5.5/pl/tablex.lua:106: attempt to assign to const variable 'k'
stack traceback:
	[C]: in ?
	[C]: in function 'require'
	[C]: in ?

Instances spotted so far (using luac5.5 -p on files):

pl/app.lua:197: attempt to assign to const variable 'k'
pl/lapp.lua:221: attempt to assign to const variable 'line'
pl/seq.lua:129: attempt to assign to const variable 'v'
pl/tablex.lua:106: attempt to assign to const variable 'k'

[Tieske]

Created #498 to fix the listed ones, but it might be incomplete. Hence waiting for 5.5 to be released so we can update CI and ensure it all works.

Meanwhile if you want to give it a go, please try #498 @Zash and report back.

[Zash]

There were some more so I opened a PR for your PR: #499