Fix AsyncFunc and some little bugs

[coveralls]

Pull Request Test Coverage Report for Build 649

• 116 of 126 (92.06%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.7%) to 85.586%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
mobula/func.py	49	51	96.08%
mobula/op/loader.py	66	74	89.19%
<!--	Total:	116	126	92.06%	-->

Files with Coverage Reduction	New Missed Lines	%
mobula/glue/backend.py	1	91.78%
mobula/glue/mx.py	1	76.07%
<!--	Total:	2		-->

Totals
Change from base Build 637:	0.7%
Covered Lines:	1431
Relevant Lines:	1672

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💛 - Coveralls