Add `LL.setdtor()` function to add a "destructor" to any Lua object.

`setdtor('description', object, function)` returns a proxy userdata object
referencing object and function. When the proxy is garbage-collected, or at
the end of the script, its destructor calls `function(object)`.

The original object may be retrieved as `proxy._target`, e.g. to pass it to
the `table` library. The proxy also has a metatable with metamethods
supporting arithmetic operations, string concatenation, length and table
indexing. For other operations, retrieve `proxy._target`. (But don't assign to
`proxy._target`. It will appear to work, in that subsequent references to
`proxy._target` will retrieve the replacement object -- however, the
destructor will still call `function(original object)`.)

Fix bugs in `lua_setfieldv()`, `lua_rawgetfield()` and `lua_rawsetfield()`.

Add C++ functions `lua_destroyuserdata()` to explicitly destroy a
`lua_emplace<T>()` userdata object, plus `lua_destroybounduserdata()`. The
latter can bind such a userdata object as an upvalue to pass to `LL.atexit()`.

Make `LL.help()` and `LL.leaphelp()` help text include the `LL.` prefix.

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