Previously we were encoding the UUID as a string, but the index it references uses a UUID
so this index can also use an encoded UUID to save a bit of memory.
Prefix queries are generally being used to match part of a partial
index. We can support these indexes by using a function that accept
different types for each subset of the index.
What I found interesting is that in the generic StringFieldIndexer the
implementation for PrefixFromArgs would remove the trailing null, but
at least in these 2 cases we actually want a null terminated string.
We simply want fewer components in the string.
These new functional indexers provide a few advantages:
1. enterprise differences can be isolated to a single function (the
indexer function), making code easier to change
2. as a consequence of (1) we no longer need to wrap all the calls to
Txn operations, making code easier to read.
3. by removing reflection we should increase the performance of all
operations.
One important change is in making all the function signatures the same.
https://blog.golang.org/errors-are-values
An extra boolean return value for SingleIndexer.FromObject is superfluous.
The error value can indicate when the index value could not be created.
By removing this extra return value we can use the same signature for both
indexer functions.
This has the nice properly of a function being usable for both indexing operations.
