Significant slowdown of HOL-ex and HOL-Codegenerator_Test

[Lawrence Paulson]

Feel free to revert, at least for those two. The original impetus was HOL-Computational_Algebra, which was taking forever to build when importing the whole of HOL-Library.

> On 13 Sep 2025, at 21:11, Makarius <makarius at sketis.net> wrote:
> 
> Above this did not quite work, notably for HOL-ex and HOL-Codegenerator_Test, where a lot of material from the parent session is actually used. Here are some timings on my AMD Ryzen 9 7950X 16-Core Processor (with threads=8).