Add caching mech to bem-walk

[qfox]

Some kind of storage class with simple interface (get/set/clean/clone/merge?).

[qfox]

Related #39

[blond]

API

I think API should look like this:

const config = require('bem-config')();
const walk = require('bem-walk');

const levelMap = config.levelMapSync();
const storage = new walk.Storage();

walk(['path/to/level'], {
    levels: levelMap,
    storage: storage
});

The Storage class should has get, set, and has methods.

const walk = require('bem-walk');
const storage = new walk.Storage();

storage.set('path/to/level', {
    'block': [
        {
            entity: 'block',
            tech: 'css',
            level: 'path/to/level',
            path: 'path/to/level/block/block.css'
        },
        {
            entity: 'block',
            tech: 'js',
            level: 'path/to/level',
            path: 'path/to/level/block/block.js'
        },
        /* ... */
    ]
});

const introspection = storage.get('path/to/level');

storage.has('path/to/level'); // true

Example:

Imagine that you need scan desktop.bundles/index and touch.bundles/index.

The first require common.blocks and desktop.blocks levels. The second require common.blocks and desktop.blocks levels.

In addition need scan lib/common.blocks for each bundle.

const config = require('bem-config')();
const walk = require('bem-walk');

const levelMap = config.levelMapSync();
const storage = new walk.Storage();

// Scan first bundle:
// the `lib/common.blocks`, `common.blocks` and `desktop.blocks` will be get from file system.
walk(['lib/common.blocks', 'common.blocks', 'desktop.blocks'], {
    levels: levelMap,
    storage: storage
});

// Scan second bundle:
// the `'lib/common.blocks` and `common.blocks` will be get from storage,
// the `touch.blocks` will be get from file system.
walk(['lib/common.blocks', 'common.blocks', 'touch.blocks'], {
    levels: config.levelMapSync(),
    storage: storage
});

// save `introspection` to file system
const introspection = storage.get('lib/common.blocks');
saveIntrospection(introspection); 

After we can load introspection:

const config = require('bem-config')();
const walk = require('bem-walk');

const levelMap = config.levelMapSync();
const storage = new walk.Storage();

// load `introspection` from file system
const introspection = loadIntrospection('lib/common.blocks');

storage.set('lib/common.blocks', introspection);

// Scan first bundle:
// the `lib/common.blocks` will be get from storage
// the `common.blocks` and `desktop.blocks` will be get from file system.
walk(['lib/common.blocks', 'common.blocks', 'desktop.blocks'], {
    levels: levelMap,
    storage: storage
});

// Scan second bundle:
// the `'lib/common.blocks` and `common.blocks` will be get from storage,
// the `touch.blocks` will be get from file system.
walk(['lib/common.blocks', 'common.blocks', 'touch.blocks'], {
    levels: config.levelMapSync(),
    storage: storage
});

Implementation

We can add storage walker and run it in core:

if (storage.has('path/to/level')) {
    storageWalker(level, add);
    callback();
} else {
    walk({ path: level, naming: naming }, add, callback);
}

[blond]

Related #14