Simplify except* implementation and move helpers to exceptions.c

[iritkatriel]

BPO	46221
Nosy	@iritkatriel
PRs	bpo-46219, 46221: simplify except* implementation following exc_info changes. Move helpers to exceptions.c. Do not assume that exception groups are truthy. #30289

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/iritkatriel'
closed_at = <Date 2022-01-02.23:23:49.017>
created_at = <Date 2022-01-01.13:59:07.040>
labels = ['interpreter-core', '3.11']
title = 'Simplify except* implementation and move helpers to exceptions.c'
updated_at = <Date 2022-01-02.23:23:49.017>
user = 'https://github.com/iritkatriel'

bugs.python.org fields:

activity = <Date 2022-01-02.23:23:49.017>
actor = 'iritkatriel'
assignee = 'iritkatriel'
closed = True
closed_date = <Date 2022-01-02.23:23:49.017>
closer = 'iritkatriel'
components = ['Interpreter Core']
creation = <Date 2022-01-01.13:59:07.040>
creator = 'iritkatriel'
dependencies = []
files = []
hgrepos = []
issue_num = 46221
keywords = ['patch']
message_count = 2.0
messages = ['409458', '409541']
nosy_count = 1.0
nosy_names = ['iritkatriel']
pr_nums = ['30289']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46221'
versions = ['Python 3.11']

[iritkatriel]

Following bpo-45711, the implementation of except* can be simplified - we do not need to use POP_EXCEPT_AND_RERAISE to reraise the exception calculated by PREP_RERAISE_STAR, we can instead

1. Make PREP_RERAISE_STAR not push a dummy lasti to the stack.
2. USE ROT_TWO + POP_EXCEPT + RERAISE.

While I'm there, I will move the PREP_RERAISE_STAR helper functions (do_reraise_star etc) to exceptions.c.

[iritkatriel]

Fixed in #30289.