Implement eval-able AnnotatedString 2-arg show

The generic/fallback AbstractString 2-arg show omits the annotations,
meaning that eval(Meta.parse(repr(::AnnotatedString))) doesn't
round-trip.

The resolution to this is fairly simple, we just need to implement a
specialised show method. The implementation is fairly obvious, we're
just also able to get away with hiding the vector type annotation since
the constructor is fine without it.

base/strings/annotated.jl

@@ -159,6 +159,22 @@ function getindex(s::AnnotatedString, i::Integer)
     end
 end
 
+# To make `AnnotatedString`s repr-evaluable, we need to override
+# the generic `AbstractString` 2-arg show method.
+
+function show(io::IO, s::A) where {A <: AnnotatedString}
+    show(io, A)
+    print(io, '(')
+    show(io, s.string)
+    print(io, ", ")
+    show(IOContext(io, :typeinfo => typeof(annotations(s))), annotations(s))
+    print(io, ')')
+end
+
+# But still use the generic `AbstractString` fallback for the 3-arg show.
+show(io::IO, ::MIME"text/plain", s::AnnotatedString) =
+    invoke(show, Tuple{IO, AbstractString}, io, s)
+
 ## AbstractChar interface ##
 
 ncodeunits(c::AnnotatedChar) = ncodeunits(c.char)

test/strings/annotated.jl

@@ -58,6 +58,9 @@
                              (3:3, :val => 2)])),
              Base.AnnotatedString("abc", [(1:2, :val => 1),
                                   (2:3, :val => 2)]))
+    @test chopprefix(sprint(show, str), "Base.") ==
+        "AnnotatedString{String}(\"some string\", , [(1:4, :thing=>0x01), (6:11, :other=>0x02), (1:11, :all=>0x03)])"
+    @test sprint(show, str, MIME("text/plain")) == "\"some string\""
 end
 
 @testset "AnnotatedChar" begin