Auto merge of #408 - servo:cells, r=jdm
typedarray: Use a Cell for handling interior mutability.
The typed array wrapper has a "computed" option that uses to cache its data.
That means that you need a mutable reference to the typed array in order to read
it, which is not great.
In particular, it's plain impossible when the typed array is provided as a
member of a struct via WebIDL.
Use a `Cell` and expose to_vec and as_slice in non-mutable references.
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