Warnings with clang when using long double for Lua floats.

In 'lcode.c', when adding constants to the list of constants of a
function, integers represent themselves in the cache and floats
with integral values get a small delta to avoid collision with
integers. (This change avoids creating artificial addresses; the old
implementation converted integers to pointers to index the cache.)

Some places don't need the "fast path" macro tointegerns, either
because speed is not essential (lcode.c) or because the value is not
supposed to be an integer already (luaV_equalobj and luaG_tointerror).
Moreover, luaV_equalobj should always use F2Ieq, even if Lua is
compiled to "round to floor".

By producing absolute line information at regular intervals, a simple
division can compute the correct entry for a given instruction.

The function 'changedline' tries harder to avoid calling
'luaG_getfuncline' plus small changes in the use of 'L->oldpc'.

Functions to set values in a table (luaH_set, luaH_newkey, etc.) receive
the new value, instead of returning a slot where to put the value.