Improving async functions

[vbcodec]

For now async functions are hardcoded to always return Task object. This is enough for most cases, but could be better to be able return Task-derived types or just any type that have specified certain members. These members are obvious now:

• passing action in constructor
• property IsCompleted
• method GetAwaiter

It could be even better to specify another optional method:
SetCallParams (Parameters As Dictionary (Of String, Object).
This method may be used to pass arguments provided to calling async function. Alternativelly there may be another keyword (for example AsyncObj), available only in async fynctions, that specify returned object (now Task).

This improvement could be helpful for implementing custom awaiters.

[ufcpp]

dup of #7169