Better magic for write_file

[Stuart Bishop]

Currently, write_file implicitly runs format in the path and the file contents. This can be useful, but can also cause great pain if the contents contains curly braces, common in many of the files we need to store like Perl or JSON or Ruby.

I propose moving the implicit format out of this helper (and others for consistency), and switching to the following syntax:

write_file(EnvFormat(path), EnvFormat(contents, foo=bar))