Feature/table row support composite keys

[sduchesneau]

example usage for composite primary key in BAYC substreams:

    events.approvals.into_iter().for_each(|evt| {
        tables
            .create_row("approval", [("evt_tx_hash", evt.evt_tx_hash),("evt_index", evt.evt_index.to_string())].into())
            .set("evt_block_time", evt.evt_block_time.unwrap())
            .set("evt_block_number", evt.evt_block_number)
            .set("approved", Hex(&evt.approved).to_string())
            .set("owner", Hex(&evt.owner).to_string())
            .set("token_id", BigDecimal::from_str(&evt.token_id).unwrap());
    });

note that we don't need the evt_tx_hash and evt_index fields to be repeated in the set(...) commands, as they are part of the primary key.

The primary could also simply be format("{}-{}", evt.evt_tx_hash, evt.evt_index).into() for a Single key.

[maoueh]

Suggested change

	* New enum in this crate: `tables::PrimaryKey`
	* Single(string): `let sng_pk: PrimaryKey = "hello world".into()`
	* Composite(BTreeMap<String, String>: `let cmp_pk: PrimaryKey = [("evt_tx_hash","hello".to_string()),("evt_index","world".to_string())].into()`
	* New enum in this crate: `tables::PrimaryKey`
	* `Single(String)`: `let single: PrimaryKey = "hello world".into()`
	* `Composite(BTreeMap<String, String>)`: `let composite: PrimaryKey = [("evt_tx_hash","hello".to_string()),("evt_index","world".to_string())].into()`

[maoueh]

Should be on the panic directly. In the sense, inline this code at line 35 directly.

[maoueh]

You got a problem with AsRef<str>?