Optimize AbstractCompositeRuntimeType storage

Optimize AbstractCompositeRuntimeType storage

Storage on schema tree does not allow for duplicates. Rather than
allocating a full Map, let's use arrays and associated binary search.
This allows us to drop the storage requirements, especially for choices,
which would end up allocating an object to contain Map.values()
representation.

This results in O(log2(N)) lookups instead of O(1), plus some additional
code, but the lookups are one-off and so well worth the reduced
footprint. A further improvement is that we relax type safety down to
Object, which allows us to not allocate singleton arrays.

Change-Id: Ice6f07a7f853bd68c614a42eda6cc1d3fd2c184e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>