This PR adds two new rules: write_file and
write_xfile.
Both rules solve a common problem: to write a text
file with user-defined contents.
The problem is routinely solved using a genrule.
That however requires Bash, since genrules execute
Bash commands. Requiring Bash is a problem on
Windows.
The new rules do not require any shell.
The only difference between the rules is that
write_xfile creates an executable file while
write_file doesn't.
See https://github.com/bazelbuild/bazel/issues/4319
Thomas Van Lenten