remove hacks in genobject.c

[iritkatriel]

Refactor Objects/genobject.c to get rid of these hacks/issues:

• gen_close accesses instr_ptr - 1
• _PyGen_yf peeks into the execution stack
• Comparisons with enum values
• result arg reused in gen_send_ex2 for two different purposes (one of which is not the result)

Linked PRs

• gh-111354: define names for RESUME oparg values #111365
• gh-111354: simplify detection of RESUME after YIELD_VALUE at except-depth 1 #111368
• gh-111354: simplify detection of RESUME after YIELD_VALUE at except-depth 1 #111459
• gh-111354: Simplify _PyGen_yf by moving some of its work to the compiler and frame state #111648
• gh-111354: remove comparisons with enum values, variable reuse, unused imports in genobject.c #111708