Add enable_if_lvalue.
Wrap RippleCalc into a single function.

* Change public members of Input and Output to remove trailing _.
* Remove Input constructor and separate flags in RippleCalc to reduce
  duplication and confusion.
* Make calculation result private; add getter.
* Narrow scope of some of the results of calls to rippleCalculate.
Bug fixes and new features for LedgerTool.

* Fix RIPD-509, RIPD-514, RIPD-519, RIPD-525, RIPD-527, RIPD-529,
  RIPD-530 and RIPD-531.
* Protect people from ledger-spew and remove cruft.
* Better error messages and handling.
* Cache command lists or clears ledger cache.
* Better ledger summaries.
* Offline mode.