URLs use special characters to indicate the parts of the request.
For example, a `?` question mark marks the end of a path and the start of a query string.
In order for that character to exist inside a path, it needs to be encoded differently.
Percent encoding replaces reserved characters with the `%` escape character
followed by a byte value as two hexadecimal digits.
For example, an ASCII space is replaced with `%20`.
When encoding, the set of characters that can (and should, for readability) be left alone
depends on the context.
The `?` question mark mentioned above is not a separator when used literally
inside of a query string, and therefore does not need to be encoded.
The [`AsciiSet`] parameter of [`percent_encode`] and [`utf8_percent_encode`]
lets callers configure this.
This crate deliberately does not provide many different sets.
Users should consider in what context the encoded string will be used,
read relevant specifications, and define their own set.
This is done by using the `add` method of an existing set.
# Examples
```
use percent_encoding::{utf8_percent_encode, AsciiSet, CONTROLS};
/// https://url.spec.whatwg.org/#fragment-percent-encode-set
const FRAGMENT: &AsciiSet = &CONTROLS.add(b' ').add(b'"').add(b'<').add(b'>').add(b'`');
assert_eq!(utf8_percent_encode("foo ", FRAGMENT).to_string(), "foo%20%3Cbar%3E");
```