A solution to I-476: Interstate Link routes

Got a little inspiration from Nebraska for a way to end the even/odd
3DI debate: add a new level of interstates for the 2DI to 2DI
routes. These link routes would preferably be only used for in-state
routes, with multi-state route staying the same (like CT-MA I-395). A
few of the routes that could be replaced:

* California: I-580 between I-80 and I-5 becomes L5C (California is
the only state in which a Interstate Link would have the same number
as a regular 2DI if one uses the same scheme as Nebraska for assigning
numbers, though Tennessee could if an I-42 pops up somewhere, and so
could Texas if it finds a way to duplicate I-43.)
* Idaho: I-86 becomes L12D (I would start with L12A if US 12 didn't
exist in that state.
* Illinois: I-155 becomes L13A
* Kansas: I-135 becomes L16A, I-335 becomes L16B, L16C is reserved in
case US 75 reaches interstate standards between the Turnpike and
Topeka, and L16D is reserved for the US 59 corridor between the
Turnpike and Lawrence.
* Michigan: I-196 becomes L22A
* Missouri: I-170 becomes L25A
* New York: I-390 becomes L32A
* Ohio: I-280 becomes L35D
* Pennsylvania: I-99 becomes L38A, I-380 becomes L38B, and I-476
becomes L38C
.

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