[draft] Improve startup and warmup performance for invoke by reducing emit usage

[steveharter]

[verifying tests]

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[janvorli]

Can you please declare the variables at the assignment below? I think it is a preferred style in the coreclr runtime and it makes it easier to see the actual type the "new" allocates.

[janvorli]

@steveharter do you plan to add support for other well known signatures than the zero arg with return value and one arg with void return ones? Based on our past discussion, we would like to eventually add ones that are on startup path of common types of apps like e.g. aspnet ones. Or were these mostly just those two basic cases?