Add b64decode

[aminalaee]

No description provided.

[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks
🆕 2 new benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	test_b64decode	N/A	97.3 µs	N/A
🆕	test_b64decode_str	N/A	82.7 µs	N/A

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_{Comparing base64decode (b4d3a63) with main (81dac3b)}