Make pins owned instead of mut borrowed

Putting PCD8544 instance along with its pins to struct is too hard due
to self-referential structs.

See also: https://stackoverflow.com/questions/32300132/why-cant-i-store-a-value-and-a-reference-to-that-value-in-the-same-struct

Initial commit, can write text to display via write!(&mut pcd8544, "...", ...), full control via functions