FS Context

[james-pre]

This PR tracks implementation of contexts, which will be used to isolate FS trees (much like chroot)

It will resolve #124.

[james-pre]

@konsumer @mcandeia,

Do you have any feedback on this feature before I merge it? For reference, you can use it like so:

import { bindContext, fs } from '@zenfs/core';
import assert from 'assert';

const one = bindContext('/root_1');

one.writeFileSync('/example.txt', 'Not in the real root.');
fs.readFileSync('/root_1/example.txt', 'utf8'); // file contents

assert.deepEqual(one.readdirSync('/../../../..'), fs.readdirSync('/root_1'));

const two = bindContext('/root_2', { uid: 100, gid: 100 });

Feel free to review the code.

[james-pre]

I've merged this PR after some feedback from Marcos. If David has any feedback, I'll handle it on the main branch.