ivy: add print unary (#170)

I held off sending this for a while, but it really is invaluable when
debugging custom operators.

doc.go

@@ -122,6 +122,7 @@ Unary operators
 	Phase                   phase   Phase of the value in the complex plane (-π to π)
 	Conjugate         +B    conj    Complex conjugate of the value
 	System functions  ⎕     sys     Argument is a string; run "sys 'help'" for details
+	Print                   print   Print and evaluate to argument; useful for debugging
 
 Binary operators
 

value/unary.go

@@ -5,6 +5,7 @@
 package value
 
 import (
+	"fmt"
 	"math/big"
 	"unicode/utf8"
 )
@@ -121,6 +122,11 @@ var IvyEval func(context Context, s string) Value
 
 var UnaryOps = make(map[string]UnaryOp)
 
+func printValue(c Context, v Value) Value {
+	fmt.Printf("%s\n\n", v.Sprint(c.Config()))
+	return v
+}
+
 func init() {
 	ops := []*unaryOp{
 		{
@@ -698,6 +704,19 @@ func init() {
 			},
 		},
 
+		{
+			name: "print",
+			fn: [numType]unaryFn{
+				intType:      printValue,
+				charType:     printValue,
+				bigIntType:   printValue,
+				bigRatType:   printValue,
+				bigFloatType: printValue,
+				vectorType:   printValue,
+				matrixType:   printValue,
+			},
+		},
+
 		{
 			name: ",",
 			fn: [numType]unaryFn{