Graduate further fastcached Core generics when a second consumer needs them

[christianparpart]

fastcached's Core has generic code that core-cpp did not import in 0.1.0: the Logger, Base64/Sha256, the Cli Options parser and EnumTable.

What is left: move each one into core-cpp when, and only when, a second consumer needs it. That is the graduation rule in .agent/rules/library-hygiene.md; one consumer's code stays in that consumer.

Recorded in .agent/rules/library-hygiene.md (Open work). Source: the plan's "Open work after v0.1.0" list in the design plan.

[christianparpart]

Reviewed during the 0.5.0 issue sweep: still governed by the graduation rule. No second consumer needs fastcached's Logger, Base64/Sha256, Cli Options or EnumTable yet (morph's base64 already moved to core::base64 in 0.1.0). Kept open as a tracker; nothing to do until a second consumer asks.