Resolve ambiguity introduced in rustc nightly 7c4b47696 2022-04-30

[blast-hardcheese]

Resolves #40

Thanks to @ashbeitz for the suggested fix.

This is noisy, but necessary to resolve the following error:

error[E0221]: ambiguous associated type `MetaData` in bounds of `K`
   --> src/indexed.rs:142:72
    |
142 |     fn as_json_with(&self, meta: impl Clone + Fn(Option<&J::MetaData>) -> K::MetaData) -> K {
    |                                                                           ^^^^^^^^^^^ ambiguous associated type `MetaData`
    |
    = note: associated type `K` could derive from `generic_json::Json`
    = note: associated type `K` could derive from `generic_json::Json`

If this is a bug in the compiler, undoing this is simply applying s/<K as generic_json::Json>/K/g and possibly undoing a handful of intentional casts that get caught in the blast. If this is a bug in json-ld, this PR can be ignored and the bug can be resolved another way.

Thank you