Backport expression caching into the Ion 1.0 readers

[zslayton]

The Ion 1.1 readers cache child expressions encountered while parsing containers and e-expressions. To do this, they use a reference to the bump allocator to create short lived values.

The Ion 1.0 readers have access to the allocator too, but do not currently make use of it. This means that parsing happens twice when reading containers, making it significantly slower than necessary.

Once this is done, the read_many_structs benchmarks need to be updated to instantiate Ion 1.0 readers instead of relying on the backwards compatibility of the more optimized 1.1 readers.