Trying to merge a wallet PR (and ending up in a rabbit hole), the latest version of the thiserror crate is implicitly referencing rust's core library, while the grin_store crate was also using the identifier core to reference grin_core. This updates grin_store's grin_core import to be explicit and resolve the error.

We probably shouldn't have used the name core, (see rust-lang/cargo#7760), but I think we'll be fine since we're exporting the crate as grin_core.

Reason for change: see dtolnay/thiserror#255

Also updates Cargo.lock