fill_cache() loads the entire Enum into memory. This should only be called for index Enums -- never for primary Enums.
Reset the Enum to the top, so that after next() is called, the Enum is located at the first term in the segment.
next() advances the state of the Enum one term. If the current position of the Enum is valid, it returns 1; when the Enum is exhausted, it returns 0.
For an Enum which has been loaded into memory, scan to the target as quickly as possible.
Setters and getters for members in the SegTermEnum struct. Not all of these are useful.