Reduce number of is_callable invocations in traverse_and_serialize_blocks

[ryanwelcher]

This PR caches the is_callable calls in traverse_and_serialize_blocks rather than calling it for each iteration of the foreach loop. This should improve the performance of the function.

Trac ticket: https://core.trac.wordpress.org/ticket/62063

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[mukeshpanchal27]

I did some profiling and found that it reduce total call count for is_callable function and reduce wall time.

Before:

After:

FYI i test TT4 blog page.

[SergeyBiryukov]

Thanks for the PR! Merged in r59077.