Improve cases where Invalid occurs at toplevel of the module

[lthls]

Ideas:

• Ensure there is always the symbol for the module block
• Change the code generation for Halt_and_catch_fire to use a C function (maybe dependent on OCAMLRUNARAM)
• Warn or error when Invalid occurs at toplevel, possibly controlled by an option

[github-actions]

This issue has been open one year with no activity. Consequently, it is being marked with the "stale" label. What this means is that the issue will be automatically closed in 30 days unless more comments are added or the "stale" label is removed. Comments that provide new information on the issue are especially welcome: is it still reproducible? did it appear in other contexts? how critical is it? etc.