Interrogation and performance improvements

This release adds a new feature to interrogation. Interrogees can now return null to any question, which indicates that the answer is unknown, not applicable, or in some other way not available. For example, calling GetDimension on a scalar wouldn't make sense, so that will now return null. This release also includes a few minor improvements to memory usage, in an effort to reduce the number of memory allocations during evaluation.

What's Changed

• Perf by @izrik in #24
• Interrogation unknown by @izrik in #25
• Prep release v0.8 by @izrik in #26

Full Changelog: v0.7...v0.8