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

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