Improve the conversion of `Callable` to a string #322

Job	Run time
Code Coverage / Code Coverage	18s
Coding Standards / Coding Standards (8.0)	27s
Dependency analysis / Dependency Analysis (8.0)	22s
Lint root / Lint yaml & Composer (8.0)	13s
Unit test / Supported Versions Matrix	5s
Static analysis / Static Code Analysis (8.0)	23s
Unit test / Tests (7.4, locked)	22s
Unit test / Tests (7.4, lowest)	22s
Unit test / Tests (7.4, highest)	25s
Unit test / Tests (8.2, lowest)	23s
Unit test / Tests (8.1, lowest)	21s
Unit test / Tests (8.1, locked)	14s
Unit test / Tests (8.0, highest)	21s
Unit test / Tests (8.2, highest)	22s
Unit test / Tests (8.0, locked)	16s
Unit test / Tests (8.1, highest)	26s
Unit test / Tests (8.4, lowest)	21s
Unit test / Tests (8.3, lowest)	34s
Unit test / Tests (8.4, locked)	14s
Unit test / Tests (8.3, highest)	28s
Unit test / Tests (8.5, locked)	18s
Unit test / Tests (8.0, lowest)	23s
Unit test / Tests (8.5, highest)	25s
Unit test / Tests (8.4, highest)	21s
Unit test / Tests (8.5, lowest)	22s
Unit test / Tests (8.2, locked)	25s
Unit test / Tests (8.3, locked)	13s
	9m 24s

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