Add #[postcard(crate = ...)] attribute for derive(Schema)

[max-heller]

Closes #183

This only applies to #[derive(Schema)] (and not #[derive(MaxSize)]) because MaxSize seems likely to disappear soon (#179) and its trait lives in a different crate (postcard::experimental::max_size vs postcard_schema). If MaxSize ends up remaining as a derive macro and is moved to postcard_schema, this attribute can work with it as well.

The syn upgrade makes it possible to use parse_nested_meta() for parsing attributes using the conventional structure of builtin attributes and is broken out into #185.

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