Update SIP-010 support

[whoabuddy]

Following the example of wrapped-nothing-v6 and the SIP-010 standard, a few more updates need to be made:

• convert all but transfer function to define-read-only
• update get-total-supply to use ft-get-balance for the FT
• create new read-only function to return contract STX balance

[314159265359879]

I think this is not going the right way because there was a mistake deploying the SIP-010 standard trait.

It should be with 'get-'

Aulneau redeployed:

@friedger @hstove so the deployed trait is wrong and we need to deploy a new one with get- prefixes?

Yes, I have deployed them both:
mainnet: https://explorer.stacks.co/txid/0x1508a980ed063e85a38728fe5caad0815e2f4ece0a1d77111185cc8c39b4b0ad?chain=mainnet
testnet: https://explorer.stacks.co/txid/0x9e57a976d429181a93ae6487c607333ca9a30789e7328294521ee05c937ac3ad?chain=testnet

Originally posted by @aulneau in stacksgov/sips#5 (comment)

[whoabuddy]

Reopened as get-balance-of is now get-balance per stacksgov/sips#5 (comment)