v1.0.0-beta.4

• Added ocaml::function macro for calling OCaml functions registered with Callback.register
• Fix spelling in book by @fmckeogh in #98
• Fixes no_std by @fmckeogh in #99
• Feature gate panic hook by @fmckeogh in #100
• Test no_std feature in CI by @fmckeogh in #101
• Implement FromValue for Box<T> by @fmckeogh in #105