Compiler: type declaration with initial value gets the value's type

[asterite]

Fixes #13023

This actually slightly changes the compiler's semantic.

Look at this code:

a = (b : Int32 = 1)
p a

What do you expect it to print?

Well, previously it printed nil. Now it prints 1. The type (and value) of a local bariable type declaration with an initial value now is the initial value's type and value.

Note: another way to have fixed this would have been to produce a nil for that expression, in the interpreter, just like compiled mode. But I think it makes more sense to change the semantics here.

For type declarations without an initial value the type is still nil:

a = (b : Int32)
a # => nil

the reason is that b doesn't really have any valid value at that point.

This also should cause any breaking change because:

1. The value of such expressions was probably rarely used.
2. And if they were used, their value was nil, so unlikely to be used for something (like, say, putting things in a collection)