Implement has() and hasMany()

Adds support of two methods:

```js
await db.put('love', 'u')
await db.has('love') // true
await db.hasMany(['love', 'hate']) // [true, false]
```

Ref: Level/community#142
Category: addition

index.js

@@ -284,6 +284,7 @@ class MemoryLevel extends AbstractLevel {
       permanence: false,
       createIfMissing: false,
       errorIfExists: false,
+      has: true,
       encodings: { [storeEncoding]: true },
       signals: {
         // Would have no value here because the operations are synchronous
@@ -313,6 +314,20 @@ class MemoryLevel extends AbstractLevel {
     return keys.map(key => this[kTree].get(key))
   }
 
+  async _has (key, options) {
+    const tree = options.snapshot != null
+      ? options.snapshot[kTree]
+      : this[kTree]
+    return tree.get(key) !== undefined
+  }
+
+  async _hasMany (keys, options) {
+    const tree = options.snapshot != null
+      ? options.snapshot[kTree]
+      : this[kTree]
+    return keys.map(has, tree)
+  }
+
   async _del (key, options) {
     this[kTree] = this[kTree].remove(key)
   }
@@ -394,3 +409,7 @@ if (typeof process !== 'undefined' && !process.browser && typeof global !== 'und
 function isRangeOption (k) {
   return rangeOptions.has(k)
 }
+
+function has (key) {
+  return this.get(key) !== undefined
+}