Modernise the tracing data structure for DynamicPPL

[mhauru]

The current main tracing data structure is VarInfo, which wraps either a Metadata or a NamedTuple with variable symbols as keys and Metadata objects as values. The latter is called a TypedVarInfo, since it allows for concrete types to be inferred for variables. In addition to the metadata field, VarInfo also holds logp and num_produce.

There's also SimpleVarInfo, which stores the values in a standard container like an OrderedDict or a NamedTuple, and only carries logp with it. It is more limited in its features, but sometimes faster.

A not-necessarily-exhaustive list of things that should be done under the banner of this roadmap issue:

• Remove unnecessary old features like Selectors and Gibbs IDs
• Refactor to simplify
• Switch from Metadata to VarNamedVector, which is simpler and has a more well-defined interface.
• Somehow generalise the types of information that VarInfo carries, i.e. not just hard-code in logp and num_produce.
• Refactor particle Gibbs so that features of VarInfo that only serve it can be removed.
• See if we could unify VarInfo and SimpleVarInfo into a best of both worlds solution to further simplify the codebase

Related issues

• Add default implementations of assume and dot_assume DynamicPPL.jl#682
• Too many gets DynamicPPL.jl#654
• Roadmap for depreciating VarInfo in favour of SimpleVarInfo  DynamicPPL.jl#416