Type variables created by the typechecker didn't have the correct
NameSpace: they were in the Var namespace rather than the TyVar
namespace, which can lead to strange warnings about quantified type
variables being not mentioned in the type when DEBUG is on.

Name:
	- added mkSystemNameEncoded for use when the string
	  is already encoded (saves re-encoding the string every
	  time)

	- added mkSystemTvNameEncoded for making a type variable
	  name, as above

Var:
	- use mkSystemTvNameEncoded when making type variables

Id:
	- add mkSysLocalUnencoded for the (rare) case when
	  the string needs encoding

TcMType:
	- use mkSystemTvNameEncoded rather than mkSystemName for
	  making type variables

SetLevels:
	- use mkSysLocalUnencoded since the names generated here
	  need encoding.