Pretty much rewritten to fulfull several long-standing wishes:

-- The whole implementation is now more table-driven.

-- Unsigned integers.  Format characters 'B', 'H', 'I' and 'L'
mean unsigned byte, short, int and long.  For 'I' and 'L', the return
value is a Python long integer if a Python plain integer can't
represent the required range (note: this is dependent on the size of
the relevant C types only, not of the sign of the actual value).

-- A new format character 's' packs/unpacks a string.  When given a
count prefix, this is the size of the string, not a repeat count like
for the other format characters; e.g. '10s' means a single 10-byte
string, while '10c' means 10 characters.  For packing, the string is
truncated or padded with null bytes as appropriate to make it fit.
For unpacking, the resulting string always has exactly the specified
number of bytes.  As a special case, '0s' means a single, empty
string (while '0c' means 0 characters).

-- Various byte order options.  The first character of the format
string determines the byte order, size and alignment, as follows:

First character		Byte order		size and alignment

	'@'		native			native
	'='		native			standard
	'<'		little-endian		standard
	'>'		big-endian		standard
	'!'		network (= big-endian)	standard

If the first character is not one of these, '@' is assumed.

Native byte order is big-endian or little-endian, depending on the
host system (e.g. Motorola and Sun are big-endian; Intel and DEC are
little-endian).

Native size and alignment are determined using the C compiler's sizeof
expression.  This is always combined with native byte order.

Standard size and alignment are as follows: no alignment is required
for any type (so you have to use pad bytes); short is 2 bytes; int and
long are 4 bytes.  In this mode, there is no support for float and
double.

Note the difference between '@' and '=': both use native byte order,
but the size and alignment of the latter is standardized.

The form '!' is available for those poor souls who can't remember
whether network byte order is big-endian or little-endian.

There is no way to indicate non-native byte order (i.e. force
byte-swapping); use the appropriate choice of '<' or '>'.