[tracking] Release 0.18

[jsdw]

• Decode BTreeMap/HashMap etc as Vec<(K,V)> #456 (Required for the release).
• Add more dynamic decoding support to subxt #406 (Large; It may be that the next release just splits the client out to pave way for a new dynamic-focused crate in a 0.18; let's see how it shapes up!) let's not try to get this in; it's probably a slightly longer term thing.
• Generate pallet RPC calls #416 (Large; requires work in substrate)
• Simplify and Streamify EventSubscription #413 (Moderate; breaking change)
• Prioritize a transaction by tip #187 (Fairly minor I think?)
• Change storage key arguments (and call arguments?) to being references #411 (Minor; small breaking change)
• Remove pub from various fields in exported types, notably in extrinsic::extra #390 (Minor; breaking change to clarify these struct APIs)
• Add more tests for events.rs/decode_and_consume_type #414 (Minor; no breaking changes)
• Address niggles from #294 #419 (Minor; no breaking changes)
• Put integration tests behind feature flag to avoid need for substrate on PATH #351 (Minor; no breaking changes)
• Update to BitVec 1.0 #422 (Minor)
• Dynamic DispatchError::Module details #443 (Minor; no breaking changes)

Nice to haves carries over from last release:

• Codegen output not implements codec::Decode #315
• Dynamically derive default mortality from System::Constants::BlockHashCount #206
• Optional start height for subscriptions #272
• Create FAQ to address common issues #393

Other thoughts:

• Static metadata validation #398 would be nice to make some progress on, even if just settling on an approach (let's see how the discussion goes in that issue before we nail it down to a particular release)

[jsdw]

Released; See #470 for the next release; I've carried over unfinished things in the top section.