Add support for numbered arrays

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I am working with a really complex binary spec (LLRP).

Some of its fields are prefixed by a size followed by that number of data 
elements (array).

It would be really useful to have an syntax to describe these elements in 
bitstring.
Consider adding the following syntax:

    SizeSpec[DataSpec]

That is, a SizeSpec followed by a DataSpec enclosed in square brackets to 
indicate that an array follows.

Examples:

    uintbe:16[bytes:8]    # A 16-bit length array of bytes.  Returns a string (i.e. "abcdef")
    uintbe:32[uintbe:32]  # A 32-bit length array of words.  Returns a list of numbers (i.e. [1,2,3,4])

It would also be great to support composite arrays.  For example:

    uint:32[uint:32, uint:16, uint:16]

    # A 32-bit length array of elements consisting of 32, 16 and 16 bit numbers.  Returns a list of lists (i.e. [[1,2,3],[4,5,6],[7,8,9]])

Finally, and I know this is a tall one, it would be great to support arrays 
inside arrays.  For example:

    uintbe:16[uintbe:8[bytes:8]]
    # A 16-bit length array of 8-bit length prefixed strings.  Returns a list of strings (i.e. ['abc', 'def'])

    uintbe:16[uintbe:32, uintbe:8[bytes:8]]
    # A 16-bit length array of elements consisting of a 32 bit number followed by a length-prefixed string.  Returns a list of lists (i.e. [[12,'Buckle my shoe'], [56, 'Pick up sticks']])

I recognize that this could complicate things *a lot*, but I figured I could 
always ask anyway.

Thanks much.  Your library has saved me days of work already.

Original issue reported on code.google.com by edward.s...@gmail.com on 6 Dec 2012 at 3:31