Improve test infrastructure

1) New method compileFiles which allows one to compile the content of a directory one file or directory
after another.

2) max constraint is printed to typr.

Added new test pos_all. Other pos tests can be retired.
Merge pull request #54 from odersky/Improve/better-tests

Improve test infrastructure
Fix of #56 - newModuleSymbol

Needs new TypeRef creation method that works for NoPrefix and at the same time
does not need a denotation. This is provided by method TermRef.withNakedSymbol.
Fix for #55

Distringuish between ModifierFlags -- which can appear in trees -- and SourceModifierFlags -- which are
modifiers according to the language spec.
Merge remote-tracking branch 'odersky/fix/#55-#56-modules' into lazyvals
Test that illustrates #55