Slightly rewrite internal C API to reflect the modern dict API

[asvetlov]

1. md_get_one(), md_get_all(), md_pop_one(), md_pop_all() now return 1 if key exists in multidict, 0 otherwise.
2. md_set_default() now also returns 1 if key exists in multidict, 0 if key doesn't exist and the default was iserted into the object.
3. All functions return -1 in case of error.

The PR doesn't affect any public API; it is a preparation step for introducing multidict's public C API.

[asvetlov]

@Vizonex FYI, I've changed md_*() functions as described in review for #1185

[codspeed-hq]

CodSpeed Performance Report

Merging #1187 will degrade performances by 14.46%

_{Comparing asvetlov:md-api (b303039) with master (184dc41)}

Summary

❌ 1 regressions
✅ 243 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_multidict_getall_str_hit[case-sensitive-pure-python-module]	99.8 µs	116.7 µs	-14.46%

[Vizonex]

@asvetlov I'll make sure that I merge these items into #1190 incase and start making changes as well as macros for the capsule api.