ctable: fix power-of-2 issue

LuaJIT dislikes pointer subtractions when the size of the pointed-to
objects is not a power of two, which leads to trace aborts. ctable
suffers from this problem. The patch pads the cdata type used as entry
for the table to the next power of two.

We also disable tail-call optimization for functions that end with a
call to a built-in, which also causes trace aborts in certain cases.

src/lib/ctable.lua

@@ -46,24 +46,28 @@ local function make_entry_type(key_type, value_type)
    else
       entry_types[key_type] = {}
    end
+   local raw_size = ffi.sizeof(key_type) + ffi.sizeof(value_type) + 4
+   local padding = 2^ceil(math.log(raw_size)/math.log(2)) - raw_size
    local ret = ffi.typeof([[struct {
          uint32_t hash;
          $ key;
          $ value;
+         uint8_t padding[$];
       } __attribute__((packed))]],
       key_type,
-      value_type)
+      value_type,
+      padding)
    entry_types[key_type][value_type] = ret
    return ret
 end
 
 local function make_entries_type(entry_type)
-   return ffi.typeof('$[?]', entry_type)
+   return (ffi.typeof('$[?]', entry_type))
 end
 
 -- hash := [0,HASH_MAX); scale := size/HASH_MAX
 local function hash_to_index(hash, scale)
-   return floor(hash*scale)
+   return (floor(hash*scale))
 end
 
 local function make_equal_fn(key_type)