[luatex] Regarding stack level elimination in code containing `tex.sprint()`

[user202729 at protonmail.com]

Currently, if there's an input level that is ended, TeX will try to remove it as soon as it's finished. For example, the following will create an infinite loop.

	```
	\def\a{\b1}
	\def\b#1{\a}
	\a
	```

Not so when `tex.cprint()` or `tex.sprint()` is involved. The following code causes a "TeX capacity exceeded" issue.

	```
	\def\a{\expandafter\b\directlua{tex.cprint(12,"1")}}
	\def\b#1{\a}
	\a
	```

This causes a problem because internally LaTeX `\char_generate:nn` macro (sometimes) uses `tex.cprint()` under the hood, and this creates issue such as https://github.com/latex3/latex3/issues/1540 .

Do you think it would be reasonable modify the engine to pop the input level (i.e. call `end_token_list()`) of type `cprint` or `sprint` ? I think this is safe because, unlike the traditional reading from file with e.g. `\input`, there is no newline character in a line created with `tex.sprint` or `tex.cprint`, so there is no good reason to keep the empty suffix of a line behind.

This may prove to be a nontrivial change however, because there are 7 calls to `end_token_list()` scattered throughout the code. (Although I think only the one call in `inputstack.c` and two calls in `expand.c` matters.)

There might be performance implication as well because of the additional checking whether the token list ended --- although this may not be as large of a concern as it appears because we can simply move the check to only when the input limit is exceeded.

What do you think?