make sure we work with unicode

[nrc]

In particular, we use byte positions where we should use char positions in many places. Furthermore, when we do use char positions, we don't check the 'physical' width of the character.

[tbu-]

Byte positions should be fine, and they're also more performant than character indices.

[vessd]

I think that rustfmt should use char positions for at least the width of the line, or writing error messages in native language leads to the fact that it is necessary to set max_width = 180.
Otherwise the line simply is not processed.

[bradjc]

When using many unicode characters in a line I get a series of:

Rustfmt failed at process.rs:894: line exceeded maximum length (sorry)

Even though my line is only 71 characters long.

[czipperz]

It appears that rust fmt replaces many non-unicode chars (such as σ or μ) with space . (test case 'μ' will be converted to ' ')

[scampi]

@czipperz Can you share an example where this happens ?