Remove CommonLayer and unused ContainerLayer methods

CommonLayer is only used by ContainerLayer, so it can be moved directly
into ContainerLayer. A vector can also be used for storing children
instead of an embedded linked-list since the operations we do on the
list of children are still very limited.
Rename ContainerLayer to Layer
No longer accept an Option for layer size

This is currently unused in Servo.
Merge pull request #76 from mrobinson/remove-common

A round of rust-layers cleanups