Changes JsonDeserializer to use ListBuffer for objects. Fixes #1487

Using a mutable.ListBuffer to deserialize large objects provides much
better performance than the existing implementation using List.
Appending to an immutable List is a linear-time operation making the
deserialization of a JSON array an O(n^2) operation.  ListBuffer offers
constant-time append and constant-time export to an immutable List,
bringing JSON object deserialization down to linear time.

Performance tests are also provided.  This test deserizalizes a 2^13
element object 100 times.