Squares are much more common than other exponentiations, and 'n*n' is
much more efficient than 'pow'.

Undo commit 6a10f03ff. Compiler performance is important, too.

That optimization was too complex and caused some weird traces when
debugging. The more common case «if ... then break» was kept.

When a stack grows, its extra area can be in use, and it becomes part
of the common area. So, the extra area must be kept correct all the
times. (Bug introduced by commit 5aa36e894f5.)

The allocation of a userdata for the state of the warn system can
cause a panic if it fails; 'luaL_ref' also can fail. This commit
re-implements the warn system so that it does not need an explicit
state. Instead, the system uses different functions to represent
the different states.

- small corrections in the manual
- ldo.c: 'docall' -> 'ccall' ('docall' already used in 'lua.c')
- comments

'get_prompt' uses 'luaL_tolstring' to convert _PROMPT or _PROMPT2
value to a string. That conversion may invoke a '__tostring'
metamethod.

Trace of vararg functions was skipping an instruction when returning
from a call. (Bug introduced by commit 5d8ce05b3.)