[Move] relax restrictions on allowed types in entrypoints

[sblackshear]

There are a few recurring requests for allowing the following types as entrypoint arguments that I think we should consider granting:

• object::ID
• ASCII::String
• string::String (i.e., UTF8 strings)

Without these, it is hard for entrypoint type signatures to fully convey the intention of the entrypoint.

[sblackshear]

CC @kchalkias @oxade

[tnowacki]

ASCII::String would be a bit tricky since it would require deserializing the argument to do the verification, which isn't something we need today.

Could also be a bit expensive (depending on the size of the data), and then not captured in gas...

[kchalkias]

Good points @tnowacki, is there a way that we enforce deserialization and verification after signature verification? so it's included in gas if it fails?

[tnowacki]

Its about metering more than anything. I think you should just manually convert/verify the string once you are in Move so you have a well metered environment, which clear semantics about the gas usage/metering.

Things like ID or even Option for that matter are much easier as they do not have the same concerns