offset() works with incompatible iterators when used with lists/&parse-[at|from]

[bbannier]

While working on #1475 I noticed that offset() seems to return incorrect results, even after pulling out the incorrect changes from #1439. The following reproducer (derived from the test added in #1477) fails in all versions down to v1.2.0.

# @TEST-EXEC: spicyc -dj %INPUT -o foo.hlto
# @TEST-EXEC: ${SCRIPTS}/printf 'FF\x01\x02' | spicy-driver foo.hlto >output 2>&1
# @TEST-EXEC: btest-diff output

module foo;

public type X = unit {
    : bytes &size=2; # FF
    : Y;
};

type Y = unit {
    payload: bytes &eod;
    : uint8[2] &parse-from=self.payload foreach { print self.offset(); }
};