Generate aliases

[badcel]

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Fixes #757

[badcel]

@cameronwhite Do you have remarks regarding the alias support? You can see it's features in the tests here.

[cameronwhite]

I think that looks fine to me.
I was wondering at first whether it should be more of a "strong typedef" that doesn't allow implicit conversions from the primitive type, if the user should mainly be using the alias type, but the implicit conversions are consistent with the C API. I don't have a strong opinion on this

[badcel]

I made it implicit as constant values can't be of an alias type as creating an instance makes them not constant. So I fall back to their base type for rendering as constant.

The implicit cast allows to use the constant primitive value as a parameter to a function which requires the alias struct.

I wonder if it is to much "magic" as I normally don't like those implicit things happening.

[cameronwhite]

That's a good point about constants, I can see that being necessary for pattern matching if there are any aliased types with constants that are useful to compare against

[badcel]

There is currently only GST_CLOCK_TIME_NONE.

It seems to work in a switch statement but not a switch expression. Perhaps there is a way forward to solve this. Will test.