Avoid allocation in ObjectIdentifier::from_str

[Rawk]

No description provided.

[RavuAlHemio]

Hmm, this change has an effect on the semantics of the obtained field of ObjectIdentifierConversionError::TooLong. Instead of setting it to index + 1, which is not necessarily true (and always ends up being MAX_SUB_IDENTIFIER_COUNT + 1), I'd either get rid of it or run through the split the OID string twice -- once to get the number of sub-identifiers, once to parse them.

What's your take?

[Rawk]

I think that it can be nice to include obtained. I have changed the commits to fix this:

• Get the obtained value by counting how many iterations are left.
• Add a test that verifies that obtained can exceed MAX_SUB_IDENTIFIER_COUNT + 1.
• Rebase.