Support nameless extensions

[warriordog]

Many AS extensions, including those used by Mastodon, add new properties onto an existing type. To support them, we should introduce the concept of "untyped extensions". This is a bit of a misnomer, as untyped extensions will still have a C# type and entity. They won't, however, have a value for the type property. They will inherit that from whichever kind of object is being extended.

Prerequisites:

• Allow entities to specify a custom context #95
• Replace ImpliesOtherEntityAttribute with an interface #108

Requirements:

• Deserialize entities without a type name #141

Related:

• Support anonymous extensions #132
• Map properties based on context #98
• Remap properties based on context #99
• Add constructor overloads to wrap an existing object #133
• Allow AS types to not have an entity #134
• Rename functions and documentation to reflect new "model" naming #142

See also:

• Trying out APSharp Letterbook/Letterbook#121