Moderately improved dict printing

This is a modest attempt at improving the situation for issue JuliaLang#1759.  I've added
slightly enhanced `summary`s with information about the number of k/v pairs for
Associative and Key/ValueIterator types.

This PR keeps the old behavior (mostly) as `showcompact`. I then added new
functionality in `show` and `showlimited`, using newlines as delimiters between
key/value pairs.  When output is limited, values are truncated at newlines or
the TTY screen edge and a limited number of pairs are printed.

The key and value iterators no longer spit out entire dictionaries when shown.
They instead show a limited {} array of the keys and values.

base/dict.jl

@@ -11,35 +11,91 @@ function in(p::(Any,Any), a::Associative)
     !is(v, secret_table_token) && (v == p[2])
 end
 
-function show{K,V}(io::IO, t::Associative{K,V})
-    if isempty(t)
-        print(io, typeof(t),"()")
-    else
+function summary(t::Associative)
+    n = length(t)
+    string(typeof(t)," with ",n,(n==1 ? " entry": " entries"))
+end
+
+function showcompact{K,V}(io::IO, t::Associative{K,V})
+    print(io,summary(t))
+    if !isempty(t)
+        print(io, ": ")
         if K === Any && V === Any
             delims = ['{','}']
         else
             delims = ['[',']']
         end
         print(io, delims[1])
         first = true
-        for (k, v) = t
+        for (k, v) in t
             first || print(io, ',')
             first = false
-            show(io, k)
+            showcompact(io, k)
             print(io, "=>")
-            show(io, v)
+            showcompact(io, v)
         end
         print(io, delims[2])
     end
 end
 
+function showdict{K,V}(io::IO, t::Associative{K,V}, limit_output::Bool = false,
+                       rows = tty_rows()-4, cols = tty_cols())
+    print(io,summary(t))
+    if !isempty(t)
+        println(io, ": ")
+        idx = 0
+        for (k, v) in t
+            idx < 1 || println(io)
+            print("  ")
+            key = sprint(show,k)
+            print(io, key)
+            print(io, " => ")
+
+            val = sprint(show,v)
+            if limit_output
+                subval = SubString(val,1,min(endof(val),cols-endof(key)-8))
+                newline = searchindex(subval,"\n")
+                lastidx = newline == 0 ? endof(subval) : newline - 1
+                if lastidx < endof(val)
+                    val = string(SubString(subval,1,lastidx),"\u2026")
+                end
+            end
+            print(io, val)
+
+            if limit_output && idx > rows - 1
+                print("\n  \u22ee => \u22ee")
+                break
+            end
+            idx += 1
+        end
+    end
+end
+
+show{K,V}(io::IO, t::Associative{K,V}) = showdict(io, t, false)
+showlimited{K,V}(io::IO, t::Associative{K,V}) = showdict(io, t, true)
+
 immutable KeyIterator{T<:Associative}
     dict::T
 end
 immutable ValueIterator{T<:Associative}
     dict::T
 end
 
+summary(iter::Union(KeyIterator,ValueIterator)) =
+    string(split(string(typeof(iter)),"{")[1]," for a ", summary(iter.dict))
+function show(io::IO, iter::KeyIterator)
+    print(io,summary(iter))
+    println(io,". Keys:")
+    print(io,"  ")
+    show(io,{k for (k,_) in iter.dict})
+end
+function show(io::IO, iter::ValueIterator)
+    print(io,summary(iter))
+    println(io,". Values:")
+    print(io,"  ")
+    show(io,{v for (_,v) in iter.dict})
+end
+
 length(v::Union(KeyIterator,ValueIterator)) = length(v.dict)
 isempty(v::Union(KeyIterator,ValueIterator)) = isempty(v.dict)
 eltype(v::KeyIterator) = eltype(v.dict)[1]