v0.2.6

This is a relatively small release; there are a few features to improve the WebAssembly demo, bug fixes and improvements for very deep Tree objects, and one more public API.

• Added VmShape serialization (using serde), specifically
  • #[derive(Serialize, Deserialize)} on VmData
  • impl From<VmData<255>> for VmShape { .. }
• Fixed stack overflows when handling very deep Tree objects
  • Added a non-recursive Drop implementation
  • Rewrote Context::import to use the heap instead of stack
• Updated Context::import to cache the TreeOp → Node mapping, which is a minor optimization (probably only relevant for unreasonably large Tree objects)
• Made fidget::render::RenderHandle public and documented