You can contract "I have" to "I've" and you can contract "have not" to "haven't".
So why not contract "I have not" to "I'ven't"?
You can also contract "I would" to "I'd" and "would not" to "wouldn't".
Why not "I would not" turn into "I'dn't"?

Because a contraction with three parts is just silly

Because that sounds too much like "Ivan" and no one likes that name.

There's no rule stopping you from doing so. Contractions are generally a by-product of degeneration in the form of the spoken word.

Do you pronounce it that way?

Quote from: TremcladClock;1381409Do you pronounce it that way?

No, just that both forms can be contracted so why not combine them?

Quote from: RomanClock;1381412No, just that both forms can be contracted so why not combine them?

There's no point in contracting it if it's not pronounced that way.

EDIT: "I'dn't" has seen use in the past, but has become unpopular due to its rarely used pronunciation. Its use can still be found in poetry.

You can apparently contract single words too.
Is "over" a waste of breath for you?  Do you hate the letter V?  No problem!  Just take that fucker out of there!