Add Array#unsafe_build

This prepares the API for refactoring array literal instantiation
without a block scope.

crystal-lang#10009

src/array.cr

@@ -153,6 +153,16 @@ class Array(T)
     ary
   end
 
+  # :nodoc:
+  #
+  # This method is used by LiteralExpander to efficiently create an Array
+  # instance from a literal.
+  def self.unsafe_build(capacity : Int) : self
+    ary = Array(T).new(capacity)
+    ary.size = capacity
+    ary
+  end
+
   # Equality. Returns `true` if each element in `self` is equal to each
   # corresponding element in *other*.
   #