Added writeBig* methods

[stampy88]

• added writeBigInt64BE, writeBigInt64LE, writeBigUInt64BE, and writeBigUInt64LE along with their aliases

[stampy88]

@dop251 wanted your thoughts on something. These tests are getting larger and larger, so I had the inclination to define some assertion utility functions in JavaScript that I could use instead of repeating similar JS. An example would be to replace something like:

name: "write with default offset",
script: `
	const buf = Buffer.alloc(16);
	const bytesWritten = buf.writeBigInt64BE(BigInt(123456789));
	
	if (bytesWritten !== 8) {
		throw new Error('bytesWritten should be 8');
	}
	
	const value = buf.readBigInt64BE(0);
	if (value !== BigInt(123456789)) {
		throw new Error('should read back the written value, got: ' + value);
	}
`,

With

name: "write with default offset",
script: `
    const buf = Buffer.alloc(16);
    assertBufferWriteRead(buf, 'writeBigInt64BE', 'readBigInt64BE', BigInt(123456789)); 
`,

My concern was that it makes it less obvious exactly what you are testing.

[stampy88]

other option would be to define finer grained assertion functions, e.g. assertBytesWritten(bytesWritten, 8) -> throws that error

[dop251]

Makes sense to me. In general I think most of the assertions need to be done in js, rather than in Go, there is even assert.js with some useful methods. Feel free to add there if you think it could be used by other packages too.

[stampy88]

Ah, I didn't even notice that assert.js file. Thanks, I'll see what I can do.

[stampy88]

@dop251 this is good to go. I'll probably do some more test refactoring on the next PR