Support infallible EnumString for enums with a default variant

[scovich]

Any enum with a #[strum(default)] variant has infallible parsing in practice, but the compiler does not know this because the error type is still strum::ParseError.

This PR proposes a non-breaking change to address the issue: Define a new type-level attribute parse_infallible, which instructs EnumString to use std::convert::Infallible for FromStr::Err, and to derive From<&str> (always) instead of TryFrom<&str> (1.34+). This allows e.g.

let variant = MyInfallibleEnum::from("some_unknown_variant")

and (in 1.82+)

let Ok(variant) = MyInfallibleEnum::from_str("some_unknown_variant");

Existing EnumString derivations remain unchanged unless the user adds the parse_infallible attribute. As a future breaking change, we could start inferring parse_infallible automatically even for enums that do not carry the annotation. If a breaking change is immediately acceptable, we don't need to define the new attribute at all, and this PR becomes simpler.

NOTE: TryFrom is still available (for 1.34+) with parse_infallible, because the From<&str> derived by EnumString activates a blanket implementation of reverse Into, which in turn activates a blanket implementation of TryFrom.

Also update docs and add new unit tests that exercise infallible parsing for both normal and PHF enums.

Fixes #307