Add initial signed-memo program

[CriesofCarrots]

The current memo program only performs UTF verification of the memo data. This facility is made more expressive in this program, which:

• Logs the memo text to the transaction log
• Requires all accounts provided to it be signers of the transaction and logs their address to the transaction log. For ease of debugging, this implementation logs a message containing the provided account address, regardless of whether the signature verification was successful

Fixes #1071

[CriesofCarrots]

As I was writing the docs page for spl.solana.com, it occurred to me it might be useful to log the UTF-8 parsing err on failure. Thoughts?

[mvines]

As I was writing the docs page for spl.solana.com, it occurred to me it might be useful to log the UTF-8 parsing err on failure. Thoughts?

Yeah, sure. Something generic like "Invalid UTF-8" sound good to help decipher an InvalidInstructionData error from transaction simulation

[mvines]

I'm leaning towards just calling this memo v2.0.0 and overwriting memo/. smemo is a superset and I'm not sure that making devs have to choose between using memo or smemo is beneficial, or having to explain to each new dev why there are two

[CriesofCarrots]

I'm leaning towards just calling this memo v2.0.0 and overwriting memo/. smemo is a superset and I'm not sure that making devs have to choose between using memo or smemo is beneficial, or having to explain to each new dev why there are two

Okay, here's what that looks like @mvines

[mvines]

Okay, here's what that looks like @mvines
👍🏼 👍🏼

[mvines]

looks super!