Implement String

[Wrench56]

Quite urgent, &str is limiting us at a lot of places. Would be great to have this. This seems like a great introductory project for me to get used to the Allocator.

[Wrench56]

• Implement RawVec
• Implement Vec
• Implement Slice

[Wrench56]

Basic AsciiString was added. Although it does not support UTF-8, we decided that ascii-based string is enough for the kernel. The implementation of AsciiString still needs some features, most depend on vector's Iterator. Therefore, I am going to leave the issue open until those features are added AND additionally Slice is reimplemented mentioned in issue #13.