raco setup does not give separate compilation guarantees for reader modules

See this issue in the #lang afl repo for a lengthier discussion about this problem.

When raco setup -j 1 compiles several files, it does not load the reader for each file in a clean environment. Instead, the read-syntax function is cached between compilations, so that compiling two files which use the same #lang foo will obtain the reader function for the first file, and subsequently re-use that function for the second file.

Unfortunately, this means that readers which use global state like those created by hygienic-reader-extension may break when the files are compiled with raco setup -j 1, but not e.g. when the files are compiled with raco setup -j 4 (as they will be compiled by different processes, which do not share state).

I think it would be better if reader modules benefited from the same kind of separate compilation guarantees that regular modules have.

Start by reproducing the behavior with raco setup -j 1 and compare it with raco setup -j 4, focusing on how reader modules and read-syntax are loaded and cached. Done means separate compilation no longer reuses reader state between files, with a regression test covering multiple files using the same #lang reader.