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 provides support for more widths, so that integers from 1 to 8 bytes (64 bits) can be accessed. The support takes two forms. Firstly, as stand-alone functions similar to the integer reading/writing methods of Buffer:

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

Read and write functions for the regular widths (8, 16, 32) are also present in this module, for consistency.

The second form is methods patched into Buffer.prototype, installed by requiring 'buffer-more-ints/polyfill':

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

buffer-more-ints/polyfill also adds methods 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')
'c01dfe'
> b.readIntLE(3, 0);
-123456

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.

Limitations

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 isContiguousInt or Buffer.isContiguousInt (polyfill) function will determine this:

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

And assertContiguousInt asserts that a number is so:

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