fix: use core not std to allow no_std to compile

[ripytide]

Getting a compile error on no_std for the new v0.26.0 release:

> cargo check
error[E0433]: failed to resolve: use of undeclared crate or module `std`
   --> strum/src/lib.rs:226:25
    |
226 | pub trait VariantArray: std::marker::Sized + 'static {
    |                         ^^^ use of undeclared crate or module `std`
    |
help: consider importing this module
    |
33  + use core::marker;
    |
help: if you import `marker`, refer to it directly
    |
226 - pub trait VariantArray: std::marker::Sized + 'static {
226 + pub trait VariantArray: marker::Sized + 'static {
    |

For more information about this error, try `rustc --explain E0433`.
error: could not compile `strum` (lib) due to 1 previous error

The fix was to use core not std.
I'm surprised this sort of thing isn't caught by CI tests?

[Peternator7]

just pushed 0.26.1 to fix it. Thanks for catching this.