It is natural for tools to take a large string concatenation like

	"1" + "1" + "1" + ... + "1"

and translate that into a sequence of go/constant calls:

	x := constant.MakeString("1")
	x = constant.BinaryOp(x, token.ADD, constant.MakeString("1"))
	x = constant.BinaryOp(x, token.ADD, constant.MakeString("1"))
	x = constant.BinaryOp(x, token.ADD, constant.MakeString("1"))
	x = constant.BinaryOp(x, token.ADD, constant.MakeString("1"))
	...

If the underlying representation of a string constant is a Go string,
then this leads to O(N²) memory for the concatenation of N strings,
allocating memory for "1", "11", "111", "1111", and so on.
This makes go/types and in particular cmd/vet run out of memory
(or at least use far too much) on machine-generated string concatenations,
such as those generated by go-bindata.

This CL allows code like the above to operate efficiently, by delaying
the evaluation of the actual string constant value until it is needed.
Now the representation of a string constant is either a string or an
explicit addition expression. The addition expression is turned into
a string the first time it is requested and then cached for future use.
This slows down the use of single strings, but analyses are likely not
dominated by that use anyway. It speeds up string concatenations,
especially large ones, significantly.

On my Mac running 32-bit code:

name               old time/op    new time/op    delta
StringAdd/1-8         160ns ± 2%     183ns ± 1%  +13.98%  (p=0.000 n=10+10)
StringAdd/4-8         650ns ± 1%     927ns ± 4%  +42.73%  (p=0.000 n=10+10)
StringAdd/16-8       3.93µs ± 1%    2.78µs ± 2%  -29.12%  (p=0.000 n=8+9)
StringAdd/64-8       37.3µs ± 9%    10.1µs ± 5%  -73.06%  (p=0.000 n=10+10)
StringAdd/256-8       513µs ± 5%      38µs ± 1%  -92.63%  (p=0.000 n=10+10)
StringAdd/1024-8     5.67ms ± 4%    0.14ms ± 2%  -97.45%  (p=0.000 n=8+10)
StringAdd/4096-8     77.1ms ± 9%     0.7ms ± 2%  -99.10%  (p=0.000 n=10+9)
StringAdd/16384-8     1.33s ± 7%     0.00s ±10%  -99.64%  (p=0.000 n=10+10)
StringAdd/65536-8     21.5s ± 4%      0.0s ± 8%  -99.89%  (p=0.000 n=10+10)

name               old alloc/op   new alloc/op   delta
StringAdd/1-8          232B ± 0%      256B ± 0%  +10.34%  (p=0.000 n=10+10)
StringAdd/4-8        1.20kB ± 0%    1.24kB ± 0%   +3.33%  (p=0.000 n=10+10)
StringAdd/16-8       14.7kB ± 0%     4.6kB ± 0%  -68.87%  (p=0.000 n=10+10)
StringAdd/64-8        223kB ± 0%      16kB ± 0%  -92.66%  (p=0.000 n=10+10)
StringAdd/256-8      3.48MB ± 0%    0.07MB ± 0%  -98.07%  (p=0.000 n=10+10)
StringAdd/1024-8     55.7MB ± 0%     0.3MB ± 0%  -99.53%  (p=0.000 n=10+10)
StringAdd/4096-8      855MB ± 0%       1MB ± 0%  -99.88%  (p=0.000 n=10+10)
StringAdd/16384-8    13.5GB ± 0%     0.0GB ± 0%  -99.97%  (p=0.000 n=9+10)
StringAdd/65536-8     215GB ± 0%       0GB ± 0%  -99.99%  (p=0.000 n=10+10)

name               old allocs/op  new allocs/op  delta
StringAdd/1-8          3.00 ± 0%      3.00 ± 0%     ~     (all equal)
StringAdd/4-8          9.00 ± 0%     11.00 ± 0%  +22.22%  (p=0.000 n=10+10)
StringAdd/16-8         33.0 ± 0%      25.0 ± 0%  -24.24%  (p=0.000 n=10+10)
StringAdd/64-8          129 ± 0%        75 ± 0%  -41.86%  (p=0.000 n=10+10)
StringAdd/256-8         513 ± 0%       269 ± 0%  -47.56%  (p=0.000 n=10+10)
StringAdd/1024-8      2.05k ± 0%     1.04k ± 0%  -49.29%  (p=0.000 n=10+10)
StringAdd/4096-8      8.19k ± 0%     4.12k ± 0%  -49.77%  (p=0.000 n=10+10)
StringAdd/16384-8     32.8k ± 0%     16.4k ± 0%  -49.97%  (p=0.000 n=9+10)
StringAdd/65536-8      131k ± 0%       66k ± 0%  -50.11%  (p=0.000 n=10+10)

https://perf.golang.org/search?q=upload:20180105.2

Fixes #23348 (originally reported as cmd/vet failures in comments on #23222).

This makes constant.Values of Kind String no longer meaningful for ==, which
required fixes in go/types. While there, also fix go/types handling of constant.Values
of Kind Int (for uint64), Float, and Complex.

Change-Id: I80867bc9c4232c5c9b213443ff16645434a68b36
Reviewed-on: https://go-review.googlesource.com/86395
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>