Add futures for calling writef with const/var string as first arg.

The compiler fails to resolve these calls. I noticed this when I was putting
together `/test/trivial/thomasvandoren/GitIntro.chpl` in commit 817119d.
Merge branch 'future-writef-with-var-pattern' of https://github.com/thomasvandoren/chapel
Add missing writef(fmt:string) versions.
Un-futurize Thomas' test.
Merge pull request #122 from mppf/writef-with-var

Writef with var fix, tests

Adds tests for writef with var/const string argument that
showed a missing overload for writef. Also adds the
missing overloads.

[reviewed by Thomas]