3005 - partial repl bugfix: trailing whitespace

Still broken: trailing comments.
It turns out this was actually fixed right at 2502, which introduced
read_all(). So this isn't some accidental regression, it's a hole
in the solution, something I just hadn't considered.

..and yes, also a hole in my methodology of applying manual tests.
3006
3007 - repl now fixed wrt manual tests
3008

Looks like my laptop was slow last night for some independent reason;
Today it takes just 1:45 to make+test.
3009

My commenting style was getting a little inconsistent; tighten up.
Current rules: no capitalization, short single incomplete sentence.
Capitalization just for macro comments at the start of a file.
Milestone comments distinguished just by '////', not uppercase.
3010