Read chains

• Fix index math generation of nested collections in CPU and RT backends.
• Collapse read chains pass.
• read_chains test.
• TODO: pass order modified to run CRC and SLF before SROA - this is fine, but is done as a hack since SROA can't handle nested products + arrays (#16 (closed)). We should still fix #16 (closed) for cases where CRC and SLF can't get rid of nested products + arrays.