```runtime: make slice growth formula a bit smoother

Instead of growing 2x for < 1024 elements and 1.25x for >= 1024 elements,
use a somewhat smoother formula for the growth factor. Start reducing
the growth factor after 256 elements, but slowly.

starting cap    growth factor
256             2.0
512             1.63
1024            1.44
2048            1.35
4096            1.30

(Note that the real growth factor, both before and now, is somewhat
larger because we round up to the next size class.)

This CL also makes the growth monotonic (larger initial capacities
make larger final capacities, which was not true before). See discussion

256 was chosen as the threshold to roughly match the total number of
reallocations when appending to eventually make a very large
slice. (We allocate smaller when appending to capacities [256,1024]
and larger with capacities [1024,...]).

Change-Id: I876df09fdc9ae911bb94e41cb62675229cb10512
Trust: Keith Randall <khr@golang.org>
Trust: Martin MÃ¶hrmann <martin@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Martin MÃ¶hrmann <martin@golang.org>
```
```diff --git a/src/reflect/value.go b/src/reflect/value.go
index 33b81d7..786c494 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
```
```@@ -2487,11 +2487,12 @@
if m == 0 {
m = extra
} else {
+		const threshold = 256
for m < i1 {
-			if i0 < 1024 {
+			if i0 < threshold {
m += m
} else {
-				m += m / 4
+				m += (m + 3*threshold) / 4
}
}
}
```
```diff --git a/src/runtime/slice.go b/src/runtime/slice.go
index e8267be..cfa862e 100644
--- a/src/runtime/slice.go
+++ b/src/runtime/slice.go
```
```@@ -185,13 +185,17 @@
if cap > doublecap {
newcap = cap
} else {
-		if old.cap < 1024 {
+		const threshold = 256
+		if old.cap < threshold {
newcap = doublecap
} else {
// Check 0 < newcap to detect overflow
// and prevent an infinite loop.
for 0 < newcap && newcap < cap {
-				newcap += newcap / 4
+				// Transition from growing 2x for small slices
+				// to growing 1.25x for large slices. This formula
+				// gives a smooth-ish transition between the two.
+				newcap += (newcap + 3*threshold) / 4
}
// Set newcap to the requested cap when
// the newcap calculation overflowed.
```