Better custom metamethod/metafield support

["Thiago L." <fakedme@...>]

I keep arguing with sandbox writers that debug.getmetatable should be 
allowed in sandboxes, even if limited to be unable to get the metatable 
of specific objects (from a blacklist table or something probably).

Something that would be really useful in Lua would be 
callmetamethod(table, "name", args), then I would be able to implement 
things like

local t = setmetatable({},{__metatable="this table is protected", 
sort__sortcmp=function() <something> return <comparator function> end})
function sort(t)
    local compfunc = callmetamethod(t,"sort__sortcmp") or <default 
comparator function>
    local sorted = {}
    <sort code>
    return sorted
end
sort(t)

even in extremely sandboxed environments...