Improve precision of return type for is.X builtins

[o-]

Here are some stats for your PR:

• WARNING: the longest CI job test_release_2 took 2.31h
• simple suite improved by 1.03
• pidigits regressed by 0.95
• Overall benchmarks improved by 1.01

Please find your performance results at https://rir-benchmarks.prl.fit.cvut.cz/diff?job_ids[]=2690452495&job_ids[]=2675292095&selection=all

[o-]

Here are some stats for your PR:

• WARNING: the longest CI job test_release_2 took 2.13h
• reversecomplement_2 regressed by 0.87
• fannkuchredux_naive regressed by 0.92
• fasta_naive regressed by 0.94
• Overall benchmarks regressed by 1.0

Please find your performance results at https://rir-benchmarks.prl.fit.cvut.cz/diff?job_ids[]=2691579881&job_ids[]=2675292095&selection=all