First, make sure that `add_methods` cannot trigger another userdata registry
insert, causing an unintended panic. Second, remove `RefCell` surrounding
userdata hashmap, as this change makes it no longer needed. Third, add a
`RefCell` around `Callback` because FnMut means that callbacks cannot recurse
into themselves, and panic appropriately when this happens. This should
eventually be turned into an error.
CallbackError now, instead of displaying the cause description, instead prints
"callback error: <traceback>". Since the cause is already in the cause chain of
the error, this avoids repeatedly printing the cause of callback errors along
the chain, and also actually prints the callback when using Display on each
error in the chain.
The stock Lua interpreter doesn't do this either. AFAIK the "base" library is supposed to be loaded into "_G" only as it contains functions like `assert` and `error`.
This cannot be accomplished without using unsafe code, which justifies this addition in my opinion.
Also changes "null" to "nul" to be in sync with `std::ffi` docs. Naming is derived from `CStr::to_bytes_with_nul`, using `as_*` instead of `to_*` since this isn't doing any computation.
Always treating warnings as errors makes development annoying ("ugh I don't want to fix this right now"), but ignoring them quickly leaves them unfixed and shines a bad light on the project when other people want build it ("hmm, this crate's a little to... yellow for my taste"). I've found that this is pretty much a perfect middle ground (even though rlua doesn't require CI to pass before merging PRs).
setmetatable now wraps a __gc method in a cclosure that aborts on error, also
'debug' library is no longer provided. We could provide just the subset of the
debug library that is sound, though.
kyren