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.
A few serialization changes coming from m-of-n development:
o Improve readability of SField.cpp.
o Better initialization of STObject.
o Trimming of STObject public methods.
o Add STObject::getFieldObject and STObject::setFieldObject.
o Make STObject::isEquivalent more robust.
o Improvements to whitespace, overrides, and virtuals.
