Use explicit conversions form 'T' to 'Optional<T>'

This commit work arounds a current defect of the emitter causing
the lowering of assignments to ignore necessary coercions for
storing the member of a union into a union container.

Author: kyouko-taiga

Tests/LibraryTests/TestCases/OptionalTests.hylo

@@ -1,11 +1,11 @@
 //- compileAndRun expecting: success
 
 public fun main() {
-  var x: Optional<Int> = 42
+  var x = 42 as Optional<Int>
   let y = if let i: Int = x { i.copy() } else { 0 }
   precondition(y == 42)
 
-  &x = None()
+  &x = None() as Optional<Int>
   let z = if let i: Int = x { i.copy() } else { 0 }
   precondition(z == 0)
 }