bpo-45417: [Enum] fix quadratic behavior during creation (GH-28907)

Creating an Enum exhibited quadratic behavior based on the number of members in three places:
- `EnumDict._member_names`: a list searched with each new member's name
- member creation: a `for` loop checking each existing member to see if new member was a duplicate
- `auto()` values: a list of all previous values in enum was copied before being sent to `_generate_next_value()`

Two of those issues have been resolved:
- `_EnumDict._member_names` is now a dictionary so lookups are fast
- member creation tries a fast value lookup before falling back to the slower `for` loop lookup

The third issue still remains, as `_generate_next_value_()` can be user-overridden and could corrupt the last values list if it were not copied.