MAINT: Refactor logic in maybe_convert_*

Creates a Seen class to abstract objects encountered when doing type
conversions.    Follow-up to pandas-dev#15005.

Author: gfyoung <[email protected]>

Closes pandas-dev#15018 from gfyoung/maybe-convert-refactor and squashes the following commits:

531a361 [gfyoung] MAINT: Refactor logic in maybe_convert_*

Author: gfyoung