Allow multi_index primary key to be a name

[nathanielhourt]

Change Description

Using a name as a primary key in a table is such a common use case that even the core contracts do it extensively. Adding first class support for this use case to the multi_index template is trivial, and allows contract code to be more expressive.

API Changes

• API Changes

The multi_index API now supports usage of a name as a table primary key, and implicitly casts it down to an int for indexing.

Documentation Additions

• Documentation Additions