Not for all operations the nesting level was incremented when needed
and/or the un-incremented nesting level was used in subfunction calls.
Binary operators such as `*` did not properly increment the nesting
level.
This PR changes `Level` in such a way that it can be used to keep track
of the nesting level when used in a loop. It is now impossible to
accidentally refer to an old nesting level value.
Resolves <https://issues.oss-fuzz.com/issues/385256115>.
