Add StringLiteral#to_utf16

closes crystal-lang#14670

spec/compiler/macro/macro_methods_spec.cr

@@ -591,6 +591,11 @@ module Crystal
         assert_macro %({{"hello world".titleize}}), %("Hello World")
       end
 
+      it "executes to_utf16" do
+        assert_macro %({{"hello".to_utf16}}), "::Slice(UInt16).literal(104_u16, 101_u16, 108_u16, 108_u16, 111_u16, 0_u16)"
+        assert_macro %({{"TEST 😐🐙 ±∀ の".to_utf16}}), "::Slice(UInt16).literal(84_u16, 69_u16, 83_u16, 84_u16, 32_u16, 55357_u16, 56848_u16, 55357_u16, 56345_u16, 32_u16, 177_u16, 8704_u16, 32_u16, 12398_u16, 0_u16)"
+      end
+
       it "executes to_i" do
         assert_macro %({{"1234".to_i}}), %(1234)
       end

src/compiler/crystal/macros/methods.cr

@@ -820,6 +820,16 @@ module Crystal
         else
           raise "StringLiteral#to_i: #{@value} is not an integer"
         end
+      when "to_utf16"
+        interpret_check_args do
+          slice = @value.to_utf16
+          # include the trailing zero that isn't counted in the slice but was
+          # generated by String#to_utf16 so the literal can be passed to C
+          # functions that expect a null terminated UInt16*
+          slice = Slice(UInt16).new(slice.to_unsafe, slice.size + 1)
+          args = slice.to_a { |codepoint| NumberLiteral.new(codepoint).as(ASTNode) }
+          Call.new(Path.global("Slice(UInt16)"), "literal", args)
+        end
       when "tr"
         interpret_check_args do |first, second|
           raise "first argument to StringLiteral#tr must be a string, not #{first.class_desc}" unless first.is_a?(StringLiteral)