at-exp and scribble: adjust reader to compose better with readtable extensions

The `at-exp` reader now delays picking up the current readtable until
`read`/`read-syntax` is called. Also, it uses the new 'dynamic configuration
of readers for the command and datum parts of an @-form, which delays a
decision of readtable on each part until reading the part.

Thanks to Alexander Knauth for sorting out pieces of the puzzle.