Add metadata uri to eth sig

[pscott]

Adds metadata URI (and support for any string) to the EIP-712 object used to authenticate users.

Basically, the idea is to use IntsSequence (represented in LE, not in BE).
On the Cairo side of things:

• Use keccak_bigend to hash the input
• Add metadata_uri_string_len as an argument to the propose function.

On the TS side of things (offchain):

• Add LEFromString method to IntsSequence (Add support for string in eip712 sx.js#48)
• Change metadata_uri to be an IntsSequence

Closes #202

[pscott]

Need to merge snapshot-labs/sx.js#48 and update the version on this PR to have it work I think :)

[Orland0x]

LGTM!