Previously we remembered the whole history of commands and replayed
them on every :load/:reload, which lead to some non-linear performance
characteristics (#5317).  The handling of the implicit Prelude import
and the implicit imports of recently loaded modules was also
complicated and wrong in various obscure ways.

The Prelude import works just like the implicit Prelude import in a
Haskell module: it can be overriden with an explicit Prelude
import.

I have added a new ":show imports" command to show which imports are
currently in force.

Prelude> :show imports
import Prelude -- implicit
Prelude> import Prelude ()
Prelude> :show imports
import Prelude ()
Prelude> map

<interactive>:0:1: Not in scope: `map'
Prelude>

Full documentation in the User's Guide.

There are various other little tweaks and improvements, such as when a
module is imported with 'as', we now show the 'as' name in the prompt
rather than the original name.