Improve IL for ValueTypes

[dadhi]

Current implementation is sub-optimal:

1. It uses Boxing where it is not always needed
2. It defines additional variable instead of using the one higher in expression tree

[dadhi]

Currently is being fixed for v1.5

[dadhi]

I've tried to fix for v1.5, but too many nuances to handle, so postponing for later. The current tests are passing, that's good.

The excessive boxing or local variable usage where it is not required affects not as many cases, therefore changing the issue to enhancement.

I have added todo: into the problematic cases, at least it makes it faster to find and understand.

[dadhi]

Ok, quick check says no more related todos.