std: Stabilize FromStr and parse
This commits adds an associated type to the `FromStr` trait representing an error payload for parses which do not succeed. The previous return value, `Option<Self>` did not allow for this form of payload. After the associated type was added, the following attributes were applied: * `FromStr` is now stable * `FromStr::Err` is now stable * `FromStr::from_str` is now stable * `StrExt::parse` is now stable * `FromStr for bool` is now stable * `FromStr for $float` is now stable * `FromStr for $integral` is now stable * Errors returned from stable `FromStr` implementations are stable * Errors implement `Display` and `Error` (both impl blocks being `#[stable]`) Closes #15138
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