Make several functions take &[T] instead of Vec[T]

The CSFML functions all copy the data given to them and do not take
ownership, so taking Vec<T> is unnecessary at best and an extra heap
allocation (for data created on the stack) at worst.

[breaking-change]
Merge pull request #44 from tomjakubowski/update-pixels-slice

Make several functions take &[T] instead of Vec[T]
improve doc
update all docs comments to respect Rust guidelines
update to respect Rust style guidelines: ' : ' -> ': '
update to last Rust
- fix lifetime