Add support for more integer widths to Buffer

buffer-more-ints: Add support for more integer widths to Buffer

Node's Buffer only supports reading and writing integers of a limited range of widths. This module extends Buffer with support for more widths, so that integers from 1 to 8 bytes (64 bits) can be accessed. The functions provided follow the same naming conventions and take the same arguments as the standard ones on Buffer:

$ node
> require('buffer-more-ints')
> new Buffer("0000deadbeef0000", "hex").readInt64BE(0).toString(16)

buffer-more-ints also adds functions readIntBE, writeIntBE, and their LE and UInt counterparts, which take an initial argument giving the width of the integer in bytes:

> var b = new Buffer(3);
> b.writeIntLE(3, -123456, 0);
> b.toString('hex')
> b.readIntLE(3, 0);

The functions added by buffer-more-ints are all implemented in terms of the core Buffer functions. Part way through writing the code, I discovered that node.js currently implements those in JavaScript, so this doesn't lead to performance benefits. But should node ever switch to implementing its Buffer operations natively, this module should get a speed boost.

As JavaScript uses IEEE754 doubles for numbers, the contiguous range of integers it can represent is [-2^53 - 1, 2^53 - 1]. So only integer widths up to 6 bytes or 48 bits can be read exactly. Reads of 7 or 8 bytes (56 or 64 bits) will return the closest value that can be represented as a JavaScript number.

In certain situations it might be important to check that a JavaScript number was read exactly. The Buffer.isContiguousInt function will determine this:

> Buffer.isContiguousInt(0x1fffffffffffff);
> Buffer.isContiguousInt(0x20000000000000);

And Buffer.assertContiguousInt asserts that a number is so:

> Buffer.assertContiguousInt(0x1fffffffffffff);
> Buffer.assertContiguousInt(0x20000000000000);
AssertionError: number cannot be represented as a contiguous integer