Use std::result::Result instead of Result

[drganjoo]

When the model contains a shape by the name of Result, the generated code fails to compile. For instance the following model fails:

@restJson1
service ConstrainedService {
    operations: [SampleOperation]
}

@http(uri: "/anOperation", method: "POST")
operation SampleOperation {
    output: SampleInputOutput
    input: SampleInputOutput
    errors: [ValidationException]
}

structure SampleInputOutput {
    result: Result
}

structure Result {
    @pattern("^a-z$")
    chat: String
}

This PR ensures:

1. If rustTemplate is being used, then #{Result} is used
2. If rust is being used, then that is changed to rustTemplate and then #{Result} is used.
3. If rustBlock is being used, then the generated code should use std::result::Result

[ysaito1001]

Thanks for going over many places! One question, is there any reason to not use *preludeScope instead of "Result" to std.resolve("result::Result")?

[drganjoo]

Thanks for going over many places! One question, is there any reason to not use *preludeScope instead of "Result" to std.resolve("result::Result")?

I maintained the use of preludeScope only in locations where it was already implemented. The array contains over 20 entries, and using *preludeScope would pass all entries onto the stack. While I initially aimed to optimize compilation time, I now realize this optimization would be negligible compared to the total compilation time. I will submit a follow-up commit to consistently use preludeScope instead of "Result" to std.resolve("result::Result")