Preserve key order when dissoc removes many keys

[Eric3-jp]

dissoc can reorder the remaining keys when removing enough keys to select its set-based strategy. For example:

from collections import OrderedDict
from toolz import dissoc

source = OrderedDict((k, str(k)) for k in range(5, -1, -1))
list(dissoc(source, 0, 1, 2, 3, factory=OrderedDict))
# Before: [4, 5]; expected: [5, 4]

Iterate the original mapping and use the existing remaining-key set only for membership checks. This preserves iteration order for dictionaries and OrderedDict factories on both sides of the size heuristic, while keeping the source unchanged. The regression includes no deletions, few deletions, many deletions, and repeated/missing keys.

Validation on Windows, CPython 3.14.0:

• New cases before the fix: 4 failed, 4 passed; afterward: all 8 passed.
• Full module tests and doctests: 277 passed, 1 skipped; 99% coverage.
• Benchmark tests: 19 passed (using the benchmark's Shakespeare fixture).
• The changed implementation passes the CI pycodestyle selection; git diff --check passes.

Prepared and validated with OpenAI Codex assistance.