[RFC] Change the default behavior of rustfmt to not to format sub-modules

[topecongiro]

Summary

Change the default behavior of rustfmt to not to format sub-modules of the given files by default. The behavior of cargo fmt will not change with this proposal.

Steps

1. Change the default behavior of rustfmt to not to format sub-modules
2. Deprecate --skip-children (as it will the default)
3. Add -r,--recursive command line option, when if present, rustfmt will format the given files and their sub-modules (the current default)

[otavio]

Given cargo fmt behavior does not change, I think it does make sense so it is, in fact, applying at the given file.