Reorganize Set, Map and descriptorPrototype types handling

[medikoo]

/cc @kamsi @mtuchowski

Currently, Set is defined via multiple: true, and Map (when we think of object with each property of same type) via _desciptorPrototype_.setProperties({ type: Object, nested: true }).

It doesn't seem intuitive, and probably can be improved, by removing both multiple: true and descriptorPrototype functions, and instead introducing aside of type meta property, an itemType property

So sets can be defined as:

Class.prototype.defineProperty('someSet', { type: db.Set, itemType: db.String })

and maps as e.g.:

Class.prototype.defineProperty('someMap', { type: db.Object, itemType: db.String })

Additionally for object kind of itemType's we may introduce (optional) itemPrototype property, which will allow to customise type provided for itemType without a need to create such type as standalone one. That thing is not possible in current implementation of dbjs (all we have is _descriptorPrototype_)

[medikoo]

Reiterating on this years later:

Ideally if object types are organized as follows:

• Map (generic object type, in current version represented by Object). Keys accept any values, Values accept any values and null. Aside provide built-in support for keyType and itemType through which keys and/or values can be restricted to specific type
• Set - Map extension, with values restricted to null type
• Object - Map extension with keys restricted to String type

Having that all multiple properties, will be reflected with property of Set type, in most cases additionally restricted through keyType (so e.g. to have list of strings, we'd need Set with keyType set to String)