Simplify internal pointer types

[pliniker]

1. Is FatPtr (interpreter/src/taggedptr.rs) necessary? Can it be eliminated?
2. Value (interpreter/src/taggedptr.rs) and TaggedScopedPtr (interpreter/src/safeptr.rs) appear to be near-identical, can we merge them?

Edit book chapter booksrc/interp-tagged-symbols.md to reflect changes.