layout: Implement `text-transform`.

[pcwalton]

The Unicode awareness of text-transform is implemented as well as
possible given the Rust standard library's Unicode support. In
particular, the notion of an alphabetic character is used instead of a
letter.

Gecko has a subclass of text run to handle text transforms, but I
implemented this in a simpler way.

r? @SimonSapin

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3445

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[pcwalton]

re-r? @SimonSapin

[SimonSapin]

r+ with some non-ASCII that is transformed added to uppercase and lowercase tests.

[pcwalton]

r=SimonSapin

[bors-servo]

saw approval from SimonSapin
at pcwalton@877f02f

[bors-servo]

merging pcwalton/servo/text-transform = 877f02f into auto

[bors-servo]

pcwalton/servo/text-transform = 877f02f merged ok, testing candidate = 9bf1d18

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/245
success: http://build.servo.org/builders/linux2/builds/245
success: http://build.servo.org/builders/android/builds/312
success: http://build.servo.org/builders/mac1/builds/462
success: http://build.servo.org/builders/mac2/builds/462

[bors-servo]

fast-forwarding master to auto = 9bf1d18