BUG: fix #3017 Inconsistency in indexing 0-d arrays with Ellipsis.
BLD: fix Bento build.

Issue was introduced with PR-3524, the ufunc override stuff.
BLD: add new test extensions (rational, struct_ufunc, operand_flag) to Bento build.
DOC: [bento] document that adding optional GCC performance improvements is TODO

[ci skip]
TST: added test for issue #3017.

Also changed style to please numpy developers.
Merge pull request #3766 from rgommers/fix-bento-build

Fix bento build
Merge pull request #3757 from dimasad/issue-3017-fix

BUG: fix #3017 Inconsistency in indexing 0-d arrays with Ellipsis.
ENH: Attempt to rewrite the index parsing.

Some optimizations still missing.
WIP: handle output dim calculation immidiatly
WIP/BUG: Add missing index DECREF.
WIP: Smaller fixes/moves
WIP: Clean up the error messages