When object has a '__name' metafield, 'luaL_tolstring' used the
received index after pushing a string on the stack.

A tail call to a C function can have the behavior of a "real" tail
call, reusing the stack frame of the caller.

The assertion cannot compute 'f->abslineinfo[i]' when the initial
estimate 'i' is -1.

The function 'isinstack' tried to work around the undefined behavior
of subtracting two pointers that do not point to the same object,
but the compiler killed to trick. (It optimizes out the safety check,
because in a correct execution it will be always true.)

When available, use the calling code to find a suitable name for what
was being called; this is particularly useful for errors of non-callable
metamethods. This commit also improved the debug information for
order metamethods.

A GC step may invoke some finalizer, which may error and emit
a warning due to stack overflfow.

- more organized handling of 'nCcalls'
- comments
- deprecation of 'setcstacklimit'

A "with stack" implementation gains too little in performance to be
worth all the noise from C-stack overflows.

This commit is almost a sketch, to test performance. There are several
pending stuff:

- review control of C-stack overflow and error messages;
- what to do with setcstacklimit;
- review comments;
- review unroll of Lua calls.