Previously it would incorrectly return `varGenericFactory` for local
variables which type was exactly the type parameter, instead of
`varFuncLocal`. This was causing local variables not to get declared in
the list of local variables.

This change brings typeparams tests to 33 pass / 98 fail.

Since we don't know the concrete type at compile time, we use a runtime
call `TypeName.zero()` to get the zero value.

In the compiler side this works in a bit of a roundabout way:
 - Statements like `var x int` are converted to `x := 0`, where zero
   value is determined by the compiler based on the variable's
   underlying type.
 - For a generic type `T` the underlying type is interface, so the
   compiler rewrites `var x T` as `x := nil // x has type T`. This is
   not entirely correct from the language specification perspective,
   because there is no universal zero value literal for a generic type
   T. However, this is the closest approximation we can express as an
   AST.
 - When compiling a nil literal into a generic-typed expression we
   assume this was meant to represent the zero value and invoke the
   runtime `T.zero()` method for the generic type T.