Allow compact ref specification

[ricekab]

Currently even very simple references are pretty long form, often taking up more than half of the actual object's definition.

An "inline" reference, similar to how id references are done, would be helpful.

A proposed solution to convert this old format:

// (normal data here)
"!refs":
  file_extension:
    target_class: FileExtension
    criteria:
      name: ".abc"

to something like:

// (normal data here)
file_extension: "!FileExtension?name=.abc#<field>"

Where the #<field> is an optional slug.

This may have some trouble with numeric types though? Might have to do field introspection to determine if a string needs to be converted to int / float / datetime / ...

[ricekab]

Implementation in 4ba55ed (tests pending).

Changed the field separation character from '#' to ':' to more clearly differentiate from id references.

[ricekab]

Fully implemented and tested in 62ce893 . Documentation is still pending, see #3

Inline reference

// General structure, field is an optional specification.
<property>: "!<class>?<field>=<value>[:<field>]"

So for example:

FileExtension: {
    "name": ".abc"
}
"File": {
    file_extension: "!FileExtension?name=.abc"
}

ID reference

Allows attaching a name to a defined entity, useful to ensure you don't "collide" with other persisted data.

FileExtension: {
    "!id": "my_extension",
    "name": ".abc"
}
"File": {
    file_extension: "#my_extension"
}