A common pattern for string_file is to want to move out the internal
string buffer, because it is the result of the computation that we want
to return. It is the reason why string_file::string returns a non-const
reference, as explained in the comment. I think it would make sense to
have a dedicated method for that instead and make string_file::string
return a const reference.
This allows removing the explicit std::move in the typical case. Note
that compile_program::compute was missing a move, meaning that the
resulting string was copied. With the new version, it's not possible to
forget to move.
Change-Id: Ieaefa35b73daa7930b2f3a26988b6e3b4121bb79
5d10a2041e