This commit removes the `ltINVALID` pseudo-type identifier from
`LedgerEntryType` and the `ttINVALID` pseudo-type identifier from
`TxType` and includes several small additional improvements that
help to simplify the code base.
It also improves the documentation `LedgerEntryType` and `TxType`,
which was all over the place, and highlights some important caveats
associated with making changes to the ledger and transaction type
identifiers.
The commit also adds a safety check to the `KnownFormats<>` class,
that will catch the the accidental reuse of format identifiers.
Ideally, this should be done at compile time but C++ does not (yet?)
allow for the sort of introspection that would enable this.
