fmt: allow %U for unsigned integers.

Suggested by jan.mercl@nic.cz.

R=rsc, jnml
CC=golang-dev
https://golang.org/cl/4376054

src/pkg/fmt/fmt_test.go

@@ -170,6 +170,7 @@ var fmttests = []struct {
 
 	// unicode format
 	{"%U", 0x1, "U+0001"},
+	{"%U", uint(0x1), "U+0001"},
 	{"%.8U", 0x2, "U+00000002"},
 	{"%U", 0x1234, "U+1234"},
 	{"%U", 0x12345, "U+12345"},

src/pkg/fmt/print.go

@@ -389,6 +389,8 @@ func (p *pp) fmtUint64(v uint64, verb int, goSyntax bool, value interface{}) {
 		p.fmt.integer(int64(v), 16, unsigned, ldigits)
 	case 'X':
 		p.fmt.integer(int64(v), 16, unsigned, udigits)
+	case 'U':
+		p.fmtUnicode(int64(v))
 	default:
 		p.badVerb(verb, value)
 	}