If we run out of things to look at in the other map, return quickly.
Also move the equal-key case above the less-than case, since maps with
equal keys are the common case when merging.
Fixing a rare case that came up in a test. In order for this to cause
a problem, the data being decoded has to have entries out of order,
and have a value that is nil or omitted.
Most maps we merge have the same keys, or at least one set of keys is
the subset of the other. Therefore, allocate a result slice capable of
holding only the max of number keys, rather than the sum.
Structs like StringLatestMap now use ps.Map directly, which saves
a memory allocation for LatestEntry.Value to point to.
The values in the ps.Map are now pointers, which saves a memory
allocation indirecting a value type to an interface{}
This commit makes the LatestMap type a sort of a base class, that
should not be used directly. This also adds a generator for LatestMap
"concrete" types with a specific data type in the LatestEntry.
