suggestion: re-structure std/log to adhere to singe-export files

[iuioiua]

Each file should export only tightly-related symbols.

Background #2936

[kt3k]

We recently sometimes choose non single-export file pattern (e.g. std/text/cases). What is the criteria to follow the pattern or divert from the pattern? Can we create an objective rule about it?

[iuioiua]

I think it's reasonable to group multiple symbols in the same file if at least one of the following is true:

1. Their signatures and dependencies have near-total overlap. E.g. std/text/cases
2. The symbols are highly likely to be used in conjunction. E.g. encoding and decoding functions.

These criteria should justify cases with many symbols within a given file or just one.

I'd be happy to document this if this sounds reasonable.

[kt3k]

Ok. Let's document them in README

[eryue0220]

Can I have a tried for this one?

[iuioiua]

Sure! I believe the next step is to factor out setup() into its own file.

[iuioiua]

Completed in #4254.