Ryn 0.1.8

Maintenance release. The version strings say 0.1.8; the compiler and the language are unchanged from 0.1.7.

• Issue and pull request templates and SUPPORT.md no longer ask for a cranelift backend or ryn bootstrap --native
  (both were removed in 0.1.6); they ask for the target and point at tools/bootstrap-native.sh.
• docs/native-ownership-validation.md, docs/native-float-validation.md and docs/language-0.2.md are marked as
  historical: they describe the Rust and Cargo era.