Propose bumping StringLimit to 128 bytes (#516)

* Propose bumping StringLimit to 64 bytes

The StringLimit of 50 bytes is too short for useful hashes of different protocols:

- When using IPFS, the hash is 46 bytes. With a protocol prefix and type prefix (IPFS vs IPNS) it becomes: `ipfs://ipfs/QmPK1s3pNYLi9ERiq3BDxKa4XosgWwFRQUydHUtz4YgpqB` or 59 bytes.
- On arweave, the hash is 43 bytes. Prefixed with just `arweave://` bumps it up to 53: `arweave://BNttzDav3jHVnNiV7nYbQv-GY0HQ-4XXsdkE5K9ylHQ`.
- On Sia, with 46 char hashes, adding `sia://` as the protocol to use will bump it past 50: `sia://GACjmEWXmYF1N3Rc-PyjN304-8M0zOXHYzAXY9222xkGhA` (52)

As such, we feel that 64 bytes is a reasonable minimum for the `StringLimit`.

* Update lib.rs

* Add different const for `UniquesStringLimit`

Co-authored-by: Shawn Tabrizi <[email protected]>