Move attribute statistics functions to stat_utils.cHEADmaster

Many of the operations done for attribute stats in attribute_stats.c
share the same logic as extended stats, as done by a patch under
discussion to add support for extended stats import and export.  All the
pieces necessary for extended statistics are moved to stats_utils.c,
which is the file where common facilities are shared for stats files.

The following renames are done:
* get_attr_stat_type() -> statatt_get_type()
* init_empty_stats_tuple() -> statatt_init_empty_tuple()
* set_stats_slot() -> statatt_set_slot()
* get_elem_stat_type() -> statatt_get_elem_type()

While on it, this commit adds more documentation for all these
functions, describing more their internals and the dependencies that
have been implied for attribute statistics.  The same concepts apply to
extended statistics, at some degree.

Author: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Yu Wang <wangyu_runtime@163.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CADkLM=dpz3KFnqP-dgJ-zvRvtjsa8UZv8wDAQdqho=qN3kX0Zg@mail.gmail.com

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