Main fn wrapping completely breaks formatting

[Noratrieb]

this is a regression from #27 (cc @bend-n if you want to fix it, don't have to of course)

i think the wrapping should continue being string based to fully preserve formatting, but using syn to detect whether wrapping should be done sounds reasonable.

[bend-n]

so we have two options
find the span of where #! starts and match it to the code, OR
rustfmt

[Noratrieb]

I do not think ferris should touch the formatting of the supplied source code. it should just wrap it in the strings with format!() as it did before

[bend-n]

yeah but format is easier