feat: basic support for Ctx{Get,Set} decorators

[tperale]

Fixed #10

Ctx decorators are a way to share informations across objects during the reading of the binary file definition.

In the following example, a streaming protocol that receives records of arbitrary length is defined.
Records have two different type a 'definition' or a 'data' both use an 'id' to identify themself. The definition defines the size of the data message they define by using CtxSet to store that size into the context. The data message uses CtxGet to fetch its size defined previously by the definition.

class RecordDefinition {
    @Relation(PrimitiveSymbol.u8)
    id: number

    @CtxSet(_ => `Definition.${_.id}`)
    @Relation(PrimitiveSymbol.u8)
    size: number
}

class RecordMessage {
    @Relation(PrimitiveSymbol.u8)
    id: number

    @CtxGet(_ => `Definition.${_.id}`)
    _size: number

    @Count('_size')
    @Relation(PrimitiveSymbol.u8)
    data
}

class Record {
    @Relation(PrimitiveSymbol.u8)
    type: number

    @Choice('type', {
        0x00: RecordDefinition,
        0x01: RecordMessage,
    })
    message: RecordDefinition | RecordMessage
}

class Protocol {
    @Until(EOF)
    records: Record[]
}