Separate base into base and prelude

Everything in prelude will be imported automatically, unless given
the --noprelude option. The idea is that what counts as "Idris" is the
compiler plus the Standard Prelude. Other libraries (base, effects, etc)
are still part of the distribution, but not necessarily standard.
Merge branch 'master' of github.com:idris-lang/Idris-dev

Conflicts:
	libs/Makefile