Changes

• BREAKING: Make JanetGc::collect an unsafe function
• BREAKING: Remove JanetEnvironment::add_def_with_doc,
  JanetEnvironment::add_var_with_doc, JanetEnvironment::add_c_func_with_doc
  and JanetClient functions with the same names
• BREAKING: Remove util::def, util::var, util::c_func
• BREAKING: Rename JanetEnviornment::add_c_func to
  JanetEnvironment::add_c_fn JanetEnvironment::add_c_fn
• Add JanetFile type
• Add JanetRng type
• Add JanetTable::try_insert and related error type
• Add DefOptions, VarOptions, CFunOptions to interact with the Janet
  environment
• Add declare_janet_mod macro to generate the machinery that Janet requires do
  create a Janet native module
  • It satisfies the same purpose as janet_mod, but it can get the
    documentation string from the function doc-comments and, for Janet versions
    above 1.17.0, it also add source map information for Janet
• Add janet_abstract::register function to register an abstract type.
• Add option to janet_fn attribute macro to include arity checks
• Add Janet::unwrap_or, Janet::unwrap_or_else and Janet::unwrap_or_default
• Implement Display for TaggedJanet and defer the Janet display
  implementation to that
• Refactor the janet_fn attribute macro parameter parsing
• Refactor the JanetEnvironment and JanetClient API
• janet_fn now emits code with the function documentation and source map
  information as constants to be used by another macro declare_janet_mod

FIxes

• Improve error report of attribute macros
• Fix compilation when no_std and with unicode feature enabled