Implement has() and hasMany()

[vweevers]

Adds support of two methods:

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

Ref: Level/community#142

[vweevers]

Optimized it by using an iterator instead of many Get() calls, and a bitset instead of many napi_set_element calls.