Fix: manifest[X].toString -> TypeStrings.fromValue

manifest[X].toString sometimes produces wacky strings that aren't valid
type names; TypeStrings.fromValue appears to behave better:
```
scala> object A
defined module A

scala> manifest[A.type].toString
res5: java.lang.String = A$@4d2aec21.type

scala> scala.tools.nsc.interpreter.TypeStrings.fromValue(A)
res6: String = A.type
```