bug#73605: [PATCH] Replace SRFI-64 with a new implementation.

[lloda]

I'm pleased to see all these fixes. However, I noticed a few breakages. They 
come from relying on undocumented behavior, but only using the public 
interface, so others might be affected. I don't propose to patch them, but 
perhaps to make a note in NEWS or (for the last two) to add a paragraph in the 
manual explaining how to achieve the same goal – the reference documentation 
doesn't have enough examples.

* test-begin and test-end now require strings. The old version accepted symbols.
* test-approximate requires real arguments. The old version accepted complex 
arguments.
* The exported variable test-log-to-file is gone.