Design decisions

Significant decisions are written down as ADRs: the problem, the decision, the alternatives and the consequences.

ADR	Decision
0001	Split into a pure Rust core and thin Python bindings
0002	Typed errors in Rust, specific exceptions in Python
0003	Durability guarantee and commit protocol
0004	Delete, update and compaction
0005	Concurrency: one open instance, thread-safe handle
0006	Relicense under Apache-2.0