Provide meaningful unit __begin value when parsing starts.

[bbannier]

We previously would not provide __begin when starting the initial parse. This meant that e.g., offset() was not usable if nothing ever got parsed.

With this patch we provide a meaningful value now.

Closes #1648.

[rsmmr]

I'm trying to recall if there was a reason for the current behavior. This is actually documented even, see https://docs.zeek.org/projects/spicy/en/latest/programming/language/types.html#method-unit::offset and other random access functions. So we will need to update the documentation at least.

[bbannier]

I'm trying to recall if there was a reason for the current behavior. This is actually documented even, see https://docs.zeek.org/projects/spicy/en/latest/programming/language/types.html#method-unit::offset and other random access functions. So we will need to update the documentation at least.

Good question. This docstring seems to have existed before we introduced the new explicit tracking of the offset value, but the parse methods still allow for parsing without specifying the a begin so it seems possible that it could still unset. I'll investigate.