v0.5.0

• Add template includes.
• Add composable RustQ splice groups.
• Use plain keyword splices for composition.
• Add structured include diagnostics.
• Add rustfmt option.
• Add generic Rust body/control-flow builders:
  • Rust.block/1
  • Rust.let_/2
  • Rust.let_mut/2
  • Rust.assign/2
  • Rust.call_stmt/3
  • Rust.return_if/2
  • Rust.if_/3
  • Rust.if_let/4
  • Rust.match_/2
• Document macro token placeholder limitation.