bpo-43892: Validate the first term of complex literal value patterns

[brandtbucher]

This fixes validation of complex literal value patterns in the parser and factors out the old compiler_pattern_capture and compiler_pattern_wildcard routines to better match the current AST.

(A few other cleanups, too.)

https://bugs.python.org/issue43892

[pablogsal]

LGTM

I checked also for refleaks (although this really doesn't add any new paths were we can leak):

❯ ./python -m test test_patma test_compile test_ast -R :
0:00:00 load avg: 1.71 Run tests sequentially
0:00:00 load avg: 1.71 [1/3] test_patma
beginning 9 repetitions
123456789
.........
0:00:00 load avg: 1.71 [2/3] test_compile
beginning 9 repetitions
123456789
.........
0:00:38 load avg: 1.40 [3/3] test_ast -- test_compile passed in 38.2 sec
beginning 9 repetitions
123456789
.........

== Tests result: SUCCESS ==

All 3 tests OK.

Total duration: 1 min 1 sec
Tests result: SUCCESS

[pablogsal]

Thanks for the cleanup, @brandtbucher ! 👌