Make LazyString mutably cache its printed value

and implement more of the string interface.

base/strings/lazy.jl

@@ -9,8 +9,10 @@ of functions).
 This type is designed to be cheap to construct at runtime, trying to offload
 as much work as possible to either the macro or later printing operations.
 """
-struct LazyString <: AbstractString
+mutable struct LazyString <: AbstractString
     parts::Tuple
+    # Created on first access
+    str::String
     LazyString(args...) = new(args)
 end
 
@@ -28,8 +30,21 @@ macro lazy_str(text)
     :(LazyString($(parts...)))
 end
 
-function print(io::IO, s::LazyString)
-    for part in s.parts
-        print(io, part)
+function String(l::LazyString)
+    if !isdefined(l, :str)
+        l.str = sprint() do io
+            for p in l.parts
+                print(io, p)
+            end
+        end
     end
+    return l.str
 end
+
+hash(s::LazyString, args...) = hash(String(s), args...)
+lastindex(s::LazyString) = lastindex(String(s))
+iterate(s::LazyString) = iterate(String(s))
+iterate(s::LazyString, i::Integer) = iterate(String(s), i)
+isequal(a::LazyString, b::LazyString) = isequal(String(a), String(b))
+==(a::LazyString, b::LazyString) = (String(a) == String(b))
+ncodeunits(s::LazyString) = ncodeunits(String(s))