Add 'deprecated' statement

[abierman]

A new statement is needed to help enforce professional API lifecycle management.

The 'deprecated' statement contains information about the associated object
which has a status of 'deprecated'

The purpose of the statement is to provide machine and human readable instructions
about the deprecation and suggest a replacement

• description: text why the node is deprecated

• replaced-by: machine-readable identifier to use instead of this identifier

• TBD: expected timeframe for obsolete status

  container foo {
      status deprecated;
      deprecated {
          description "This node replaced by new system module";
          replaced-by bar;
      }
  }
  

[BalazsLengyel]

Support it.

[avtobiff]

Support this.