Adds bugfixes for generated code

[desaikd]

Issue #88:

Description of changes:

This PR includes the first prerequisites of #88. This PR adds some bug fixes for the generated code.

List of changes:

• First commit adds few bug fixes of the generated rust and java code. These changes are now inside code-gen-core.
  • changed the template for rust struct.
  • add is_root_type in CodeGenerator, this will be used for stepping in for root and type for consecutive calls just calling next. (Note: This will be removed once we have Lazy Reader APIs)
  • Added a boolean parameter to AbstractDataType::Struct that will be used by template to check if the generated struct is closed or can contain open content. (Note: This just sets up the parameter to the enum but is not yet being used by template, those changes will arrive in later PRs.)
  • Field entry that is used by template now stores the ISL type name as that will be used to determine calling read/write_sym/string, blob/clob etc.
  • changed java/class.templ to properly case properties of a class and adds a minor change for constructor signature.

Tests:

Still uses the older tests in ion-cli but will be refactored in following PRs to use new roundtrip tests.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[desaikd]

As per an offline discussion, there is a possible solution for generated code testing without the need to create code-gen-core. Retracting the workspace set up PR, will open up a separate PR that introduces the new solution for generated code's testing.

[popematt]

Is it possible to make the generated types implement WriteAsIon? I think that will make it easier overall, especially since there's already things like impl <T> WriteAsIon for Vec<T> where T: WriteAsIon.