Use the new infix synonym for mappend (<>)

Define it in Compat to support base < 4.5

Also, tweak Compat's doc comment, as we no longer use regex (see a2a04b3d).
Add test/Fold.hs

If we switch to PostgreSQL parameterized queries in the future,
this will break:

    query conn "FETCH FORWARD ? FROM fold" (Only chunkSize)

PostgreSQL doesn't support parameter substitution here:

    > FETCH FORWARD $1 FROM c;
    ERROR:  syntax error at or near "$1"
    LINE 1: FETCH FORWARD $1 FROM c;

postgresql-simple constructs the query string itself, so this
works for now.