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Fixes scalameta#1720.

FormatWriter: {} => (): don't rewrite some lambdas

We can't rewrite a lambda unless it has parens around parameters or one
simple parameter without a type.

Fixes scalameta#1707.
Fixes scalameta#1708.

FormatWriter bugfix: no commas if preserve, no NL

Rewrites: traverse tree once, applying all rules

Also, turn rewrites into sessions with the context instance.

Merge pull request scalameta#1656 from tanishiking/enable-to-downlad-…

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