reverse-string: encourage grapheme awareness (#261)

Closes #238

exercises/reverse-string/README.md

@@ -6,6 +6,19 @@ For example:
 input: "cool"
 output: "looc"
 
+## Bonus
+
+Preserve grapheme clusters, i.e.
+
+```julia
+myreverse("hi 👋🏾") == "👋🏾 ih"
+myreverse("as⃝df̅") == "f̅ds⃝a"
+```
+
+You will probably find the `Unicode` stdlib useful for this bonus task.
+
+To enable the graphemes test, add `const TEST_GRAPHEMES = true` to the global scope of your file.
+
 ## Source
 
 Introductory challenge to reverse an input string [https://medium.freecodecamp.org/how-to-reverse-a-string-in-javascript-in-3-different-ways-75e4763c68cb](https://medium.freecodecamp.org/how-to-reverse-a-string-in-javascript-in-3-different-ways-75e4763c68cb)

exercises/reverse-string/example.jl

@@ -1,7 +1,7 @@
+using Unicode: graphemes
+
+const TEST_GRAPHEMES = true
+
 function myreverse(phrase::String)
-    chars = Char[]
-    for c in phrase
-        push!(chars, c)
-    end
-    join(chars[end:-1:1])
+    join(reverse(collect(graphemes(phrase))))
 end

exercises/reverse-string/runtests.jl

@@ -32,3 +32,14 @@ end
 @testset "reversing a string twice" begin
     @test myreverse(myreverse("gift")) == "gift"
 end
+
+@testset "emoji" begin
+    @test myreverse("hi 🐱") == "🐱 ih"
+end
+
+if @isdefined(TEST_GRAPHEMES)
+    @eval @testset "graphemes" begin
+        @test myreverse("as⃝df̅") == "f̅ds⃝a"
+        @test myreverse("hi 👋🏾") == "👋🏾 ih"
+    end
+end