Add field-access-same-line config option

[kangalio]

Resolves #3641.

Adds a new configuration option field-access-same-line, which puts subsequent field accesses in chains on the same line. For example:

self.serializer
    .writer
    .write_u32::<LittleEndian>(array_offset)?;

turns into

self.serializer.writer
    .write_u32::<LittleEndian>(array_offset)?;

A more elaborate test case is in the tests directory.

Field accesses with indexing (e.g. self.field[5].field) in them are not correctly put on a single line, because, apparently, rustfmt splits chains into multiple separate Chain objects when there is indexing in it. I don't know the motivation behind this and I think it's too complicated for me as a newcomer to the codebase to fix.

[calebcartwright]

See #4888 (comment) for more info