Update ascii to version 0.7

[tomprogrammer]

These are some rather simple changes to update the ascii crate to the latest version.

[frewsxcv]

It looks like examples/serve-root.rs relies on the old ascii version. Could you update that file as well? If not I can update it later

[tomprogrammer]

I force-pushed the changes. Somehow I forgot to check the examples and tests..

[tomaka]

It looks like ascii is now part of the stdlib, except that I'm not sure if all the features are present. If so, then we should eventually use it instead.
But for now that PR looks good.

[tomprogrammer]

Yes, the std::ascii::AsciiExt trait is for "ASCII-subset only operations on string slices". The difference between the ascii crate and the trait in the standard library is, that ascii can guarantee the encoding of the entire string, like str guarantees it for UTF-8.

The Header struct encodes the requirement of ascii-only values in the type AsciiString for the value field. When the AsciiExt trait is used, there have to be more checks throughout the code.
Things like php.stderr.clone().as_ascii_str() certainly could be rewritten using AsciiExt::is_ascii().

[frewsxcv]

Thanks!