fmt.Print*: reimplement to switch on type first.

This shortens, simplifies and regularizes the code significantly.
(Improvements to reflect could make another step.)
Passes all.bash.

One semantic change occurs: The String() method changes
behavior. It used to run only for string formats such as %s and %q.
Instead, it now runs whenever the item has the method and the
result is then processed by the format as a string. Besides the
regularization, this has three effects:

	1) width is honored for String() items
	2) %x works for String() items
	3) implementations of String that merely recur will recur forever

Regarding point 3, example from the updated documentation:
	type X int
	func (x X) String() string { return Sprintf("%d", x) }
should cast the value before recurring:
	func (x X) String() string { return Sprintf("%d", int(x)) }

R=rsc
CC=golang-dev
https://golang.org/cl/1613045

1 files changed, 1 insertions, 1 deletions

diff --git a/test/const3.go b/test/const3.go
index d49df2b88f..dd5c88958d 100644
--- a/test/const3.go
+++ b/test/const3.go
@@ -10,7 +10,7 @@ import "fmt"
 
 type T int
 
-func (t T) String() string { return fmt.Sprintf("T%d", t) }
+func (t T) String() string { return fmt.Sprintf("T%d", int(t)) }
 
 const (
 	A T = 1 << (1 << iota)