Some classes had virtual methods, but were missing a virtual
destructor.
Technically, every unit test that inherits from the Beast test suite
would get flagged by `-Wnon-virtual-dtor` but I did not think it would
be a great idea to go sprinkle a virtual destructor for every Ripple
test suite.
Add support for the SignerListSet transaction as a step toward
multi-sign support.
As part of the SignerListSet implementation, add InnerObjectFormat
templates (similar to TxFormats and LedgerFormats) and enforce them
in STObject, STArray, and STParsedJSON.
