encoding: Optimized encoding of Interest and related data structures

Change-Id: I0609b40565835568e09c3cc0330db441fd9243b6
refs: #1172
encoding: Optimized Data packet encoding, preparation for memory-efficient signing operation

Change-Id: I6eb1f8acef917970790d1f228ade6212c45181fa
refs: #1172
helpers: Several optimizations with command interest helpers

Both command interest helpers are now header only.   TestCommandInterest
is combined with TestSignedInterest as a second test case under the same
test suite.

Change-Id: Ieb80b24d76ded516a3d40770b65fcc1afd3e00a5