Test Fwk: Add conversions for ScVal<=>Into<RawVal>

[leighmcculloch]

We should add a impl TryFrom<ScVal> for RawVal that can convert an ScVal to a RawVal. It wouldn't be able to support all types but it could support all but the SCV_OBJECT arm.

For the XDR types that parallel types that are RawValConvertible we should also add a impl From<_> for RawVal. This would be for any arm that has a non-builtin type. i.e. SCV_STATIC, SCV_SYMBOL, and SCV_STATUS.

We can also implement conversions in the opposite direction and most will be symmetrical, except the SCV_POS_I64 arm that would not be able to convert the other way for negative values of int64.

@jonjove asked for how to do these conversions in a meeting today. @graydon suggested we should have this in the common package.

cc @jonjove @graydon

[leighmcculloch]

Actually, I just remember, I took a stab at implementing this previously:

• Move ScVal<>RawVal conversion to common #81

@graydon you and I discussed it in Slack (ref), and you had said to me that you preferred it not to escape the host. Do you still agree that these conversions shouldn't escape the host, or okay for me to revive that PR?

[leighmcculloch]

@jonjove What were you needing this for?

[graydon]

I want the WeakHost type not to escape the host crate. I'm not as concerned about the general concept of conversion, though I suspect it will confuse users that some-but-not-all SCVals can be converted without an Env.

[leighmcculloch]

I want the WeakHost type not to escape the host crate. I'm not as concerned about the general concept of conversion

Ah ok, I think I misunderstood your prior comment.

I suspect it will confuse users that some-but-not-all SCVals can be converted without an Env.

I planned to only implement the to and from EnvVal conversions, not the RawVal.

I'll revive #81 then.

[leighmcculloch]

If possible we should make this for conversions from ScVal from/to IntoEnvVal<E, V> so that we can seamless convert SDK types to and from ScVal types. This is requires for making stellar/rs-soroban-sdk#240 usable on all types of UDTs, since UDTs can contain SDK types.