fix: `impl<'a> From<AsciiString> for Cow<'a, AsciiStr>`

[DanikVitek]

Fixes #110

Modify the implementation of From<AsciiString> for Cow<'a, AsciiStr>.

• Change impl From<AsciiString> for Cow<'static, AsciiStr> to impl<'a> From<AsciiString> for Cow<'a, AsciiStr> in src/ascii_string.rs
• Broaden the conversion abilities by making it generic over any lifetime 'a

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[DanikVitek]

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