Revert #71372 ("Fix #! (shebang) stripping account space issue").

[eddyb]

While #71372 fixed some of the problems #!-stripping had, it introduced others:

• inefficient implementation (.chars().filter(...).collect() on the entire input file)
  • this also means the length returned isn't always correct, leading to e.g. thread 'rustc' panicked at 'begin <= end (2 <= 0) when slicing  #71471
• it ignores whitespace anywhere, stripping # ! ... which isn't a valid shebang
  • the definition of "whitespace" it uses includes newlines, which means even \n#\n!\n... is stripped as a shebang (and anything matching the regex \s*#\s*!\s*, and not followed by [, really)
• it's backward-incompatible but didn't go through Crater

Now, #71487 is already open and will solve all of these issues. But for running Crater, and just in case #71487 takes a bit longer, I decided it's safer to just revert #71372.

This will also make #71372's diff clearer, as it will start again from the original whitespace-unaware version.

r? @petrochenkov

[petrochenkov]

@bors r+ rollup

[bors]

📌 Commit 4d67c8d has been approved by petrochenkov

[petrochenkov]

Did #71372 go into beta, btw?
UPD: Looks like it didn't.