Library changes: Please read details below!

Builtins now contains only those things required to support builtin
syntax. Everything else moved into Prelude, which has been reorganised
a little (extra files for Bool, Classes and Basic functions such as
id and composition).

New --nobuiltins flag, which is intended only to built the Builtins
module. --noprelude now loads Builtins. --nobuiltins implies
--noprelude.

NOTE: You may need to delete the libs/ directory and check it out again
to be sure there are no stale ibc files (we really need to fix the
build process!)
Remove needless Prelude imports
Add --nobasepkgs flag

Intended for use when building the base packages, so that they don't
look at installed versions. Simply removes 'prelude' and 'base' from
the package list.