These changes reduce sizeof(match_context):

• 32-bit build: 36 bytes, no change.
• 64-bit build: 72 bytes -> 56 bytes.

sre uses stack and match_context struct to simulate recursive call, smaller struct brings:
- deeper recursive call
- less memory consume
- less memory realloc

Here is a test, if limit the stack size to 1 GiB, the max available value of n is:

re.match(r'(ab)*', n * 'ab')   # need to save MARKs
72 bytes: n = 11,184,808
64 bytes: n = 12,201,609
56 bytes: n = 13,421,770

re.match(r'(?:ab)*', n * 'ab') # no need to save MARKs
72 bytes: n = 13,421,770
64 bytes: n = 14,913,078
56 bytes: n = 16,777,213

1,073,741,823 capturing groups should enough for almost all users.
If limit it to 16,383 (2-byte integer), the context size may reduce more. But maybe some patterns generated by program will have more than this number of capturing groups.

1️⃣Performance:

Before
regex_dna: Mean +- std dev: 149 ms +- 1 ms
regex_effbot: Mean +- std dev: 2.22 ms +- 0.02 ms
regex_v8: Mean +- std dev: 22.3 ms +- 0.1 ms
my benchmark[1]: 13.9 sec +- 0.0 sec

Commit 1. limit the maximum capture group to 1,073,741,823
regex_dna: Mean +- std dev: 150 ms +- 1 ms
regex_effbot: Mean +- std dev: 2.16 ms +- 0.02 ms
regex_v8: Mean +- std dev: 22.3 ms +- 0.1 ms
my benchmark: 13.8 sec +- 0.0 sec

Commit 2. further reduce sizeof(SRE(match_context))
regex_dna: Mean +- std dev: 150 ms +- 1 ms
regex_effbot: Mean +- std dev: 2.16 ms +- 0.02 ms
regex_v8: Mean +- std dev: 22.2 ms +- 0.1 ms
my benchmark: 13.8 sec +- 0.1 sec

If further change the types of toplevel/jump from int to char, in 32-bit build sizeof(match_context) will be reduced from 36 to 32 (In 64-bit build still 56). But it's slower on 64-bit build, so I didn't adopt it:
regex_dna: Mean +- std dev: 150 ms +- 1 ms
regex_effbot: Mean +- std dev: 2.18 ms +- 0.01 ms
regex_v8: Mean +- std dev: 22.4 ms +- 0.1 ms
my benchmark: 14.1 sec +- 0.0 sec

2️⃣ The type of match_context.count is Py_ssize_t
- If change it to 4-byte integer, need to modify some engine code.
- If keep it as Py_ssize_t, SRE_MAXREPEAT may >= 4 GiB in future versions.
Currently SRE_MAXREPEAT can't >= 4 GiB.
So the type of match_context.count is unchanged.

[1] My re benchmark, it uses 16 patterns to process 100 MiB text data:
https://github.com/animalize/re_benchmarks