This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.
One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
Also, add `Line::to_type<T>()` since that was missing.
Calling to_type() with the same type as the existing object accomplishes
nothing except wasting some cycles and making the code more verbose,
and it is hard to spot. Nobody does this in the code currently
(yay!) but I made this mistake repeatedly when doing my step-by-step
CSS Pixels conversion, so let's make it easier to catch them.
AK's version should see better inlining behaviors, than the LibM one.
We avoid mixed usage for now though.
Also clean up some stale math includes and improper floatingpoint usage.
These helpers will be useful in preparation for supporting multiple
displays, e.g. to measure distances to other screens or figure out
where rectangles are located relative to each other.
