a few small fixes for codegen

[ExpandingMan]

This PR includes a few small fixes;

• I have added the reserved names Array, Vector, Dict, Union and OneOf. These are names which, if used, would break the generated code because their Base names are used without any accommodation to qualify the names in those cases. Ideally I'd like to add a whole lot more (every exported type from Base) as they can lead to nasty surprises even if they don't technically break the generated code, but technically adding any other reserved words would be breaking, whereas this PR is not.
• I fixed an edge case in which a space would be omitted when generating code for a 0-field struct which inherits from AbstractProtoBufMessage. This would lead to broken code such as struct A <: AbstractProtoBufMessageend. I have added a unit test for this case.
• using EnumX in code generation is now using ProtoBuf.EnumX which uses the EnumX belonging to ProtoBuf obviating the need for packages using protobuf to include EnumX in their environment. Using indirect dependencies is a discouraged practice, however I think it makes sense in this case since the generated code is a bit like internal module code in that it may depend on internal details of the module.

[codecov[bot]]

Codecov Report

Base: 89.04% // Head: 89.04% // No change to project coverage :thumbsup:

Coverage data is based on head (a6577ca) compared to base (787898a). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #221 +/- ## ======================================= Coverage 89.04% 89.04% ======================================= Files 25 25 Lines 2794 2794 ======================================= Hits 2488 2488 Misses 306 306 ``` | [Impacted Files](https://codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/codegen/names.jl](https://codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/221/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2NvZGVnZW4vbmFtZXMuamw=) | `100.00% <100.00%> (ø)` | | | [src/codegen/toplevel\_definitions.jl](https://codecov.io/gh/JuliaIO/ProtoBuf.jl/pull/221/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2NvZGVnZW4vdG9wbGV2ZWxfZGVmaW5pdGlvbnMuamw=) | `96.50% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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[Drvi]

Thanks for your contribution! Nice improvement:)