Still missing:
- handling of memory errors when creating upvalue (must run closing
method all the same)
- interaction with coroutines

Start of the implementation of "scoped variables" or "to be closed"
variables, local variables whose '__close' (or themselves) are called
when they go out of scope. This commit implements the syntax, the
opcode, and the creation of the corresponding upvalue, but it still
does not call the finalizations when the variable goes out of scope
(the most important part).

Currently, the syntax is 'local scoped name = exp', but that will
probably change.

Resuming a coroutine uses more C stack than other operations (such as
function calls or recursive syntax). So, to avoid stack overflow
in recursive coroutine invocations, either LUAI_MAXCCALLS must be
too small or a coroutine invocation must "pay" a higher price.
New constant LUAL_COROCSTK ("COROutine C STaK") defines how much
is this price.

The multiplication (m*b) used to test whether 'm' is non-zero and
'm' and 'b' have different signs can underflow for very small numbers,
giving a wrong result. The use of explicit comparisons solves this
problem. This commit also adds several new tests for '%' (both for
floats and for integers) to exercise more corner cases, such as
very large and very small values.

As hinted in the manual for Lua 5.3, the emulation of the metamethod
for '__le' using '__le' has been deprecated. It is slow, complicates
the logic, and it is easy to avoid this emulation by defining a proper
'__le' function.

Moreover, often this emulation was used wrongly, with a programmer
assuming that an order is total when it is not (e.g., NaN in
floating-point numbers).

Version numbers and dates (mostly wrong) from RCS keyword strings
removed from all source files; only the file name are kept.

Minor optimizations in 'lvm.c'. (Not exactly optimizations, but more
chances for optimizations.)