Add a Schema impl for std::string::String with the use-std feature and for alloc::string::String with alloc

[zRedShift]

Hi, we found a use for Schema in an std context, and the non-heapless String impl was missing.

Would you be okay with adding this impl?

BTW, I find that that this experimental feature is useful for serde in general, not just for postcard use.

[netlify]

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[jamesmunns]

Looks great, thanks!