Implements new feature that removes nested blocks#5616.
The feature is similar toremove_nested_parens. Blocks with any sort of comments or attributes,unsafe andconst blocks will not be removed. This however will not remove all nested blocks, instead it will leave the innermost block (similar toremove_nested_parens). That behavior differs from suggested in the issue, and I'm not sure which one is better.

I'm not fully satisfied with my implementation, so any suggestions are appreciated.
I also was not sure what do do if nested blocks are of different kind (nestedconst and/orunsafe blocks), so I decided to only remove 'naked' blocks, and just never removeconst andunsafe blocks. There is some merit to removingunsafe orconst blocks when only blocks of one kind are nested, but for the sake of consistency I decided to not do that. An example of such problematic situation:

// rustfmt-remove_nested_blocks: truefnfoo(){}fnone_kind(){{// <- this block would be removed if not for this commentconst{const{}}}}fnmixed(){unsafe{unsafe{{// <- this block would be removed if not for this commentconst{const{{// this block will not be removed regardless of the commentfoo();}}}}}}}

As I said, for now bothmixed, andone_kind functions would only remove the blocks annotated by comments.
I'm open for suggestions regarding the behavior, since the feature in the original issue was very loosely defined (also blocks are more tricky than parentheses)