Fix: main function does not check inferred return type nor namespacevar type

[Pablo96]

What does this PR do?

It fixes issue 483

Description of the bug it fixes

On the main function, given no explicit return value it does not check the inferred type against is_integer, it is related to another bug I found where if the expression is a NamespacedVar it does not check for its type either.

[BuggieBot]

Hello!

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[awesomekling]

This is still failing rustfmt on CI. Also, please squash the fixup into the original commit instead of adding a separate commit :)

[AtkinsSJ]

Closed as this is a change for the now-gone Rust-based compiler. Feel free to port your changes to the new compiler, if they're not already added, and open a new PR. :^)