runtime: move whenHeap into heap slice

runtime: move whenHeap into heap slice

This CL changes the timer heap from a slice of timers to a slice
of {timer, when} pairs, moving timer.whenHeap into the heap
backing store itself. This makes the ownership clearer (t.whenHeap
was always protected by the heap lock, not the timer's lock)
and also avoids an indirection when doing heap operations,
which look at the when field quite a lot.

goos: linux
goarch: amd64
pkg: time
cpu: AMD Ryzen 9 7950X 16-Core Processor
                                   │  s7base.txt  │                s7.txt                │
                                   │    sec/op    │    sec/op     vs base                │
AdjustTimers10000-32                 244.1µ ±  5%   195.9µ ± 13%  -19.76% (p=0.001 n=10)
AdjustTimers10000SingleThread-32     1.674m ±  6%   1.573m ± 13%   -6.03% (p=0.001 n=10)
AdjustTimers10000NoReset-32          194.0µ ±  2%   170.6µ ±  1%  -12.06% (p=0.000 n=10)
AdjustTimers10000NoSleep-32          223.0µ ±  7%   183.9µ ±  2%  -17.54% (p=0.000 n=10)
AdjustTimers10000NoResetNoSleep-32   181.1µ ±  1%   151.3µ ±  1%  -16.45% (p=0.000 n=10)

goos: darwin
goarch: arm64
pkg: time
cpu: Apple M3 Pro
                                   │ m3base.txt  │               m3.txt                │
                                   │   sec/op    │   sec/op     vs base                │
AdjustTimers10000-12                 266.6µ ± 2%   234.2µ ± 1%  -12.16% (p=0.000 n=10)
AdjustTimers10000SingleThread-12     1.194m ± 0%   1.191m ± 1%   -0.33% (p=0.029 n=10)
AdjustTimers10000NoReset-12          260.9µ ± 2%   239.6µ ± 2%   -8.15% (p=0.000 n=10)
AdjustTimers10000NoSleep-12          247.8µ ± 2%   223.3µ ± 2%   -9.90% (p=0.000 n=10)
AdjustTimers10000NoResetNoSleep-12   231.7µ ± 2%   209.2µ ± 2%   -9.68% (p=0.000 n=10)

Change-Id: I546f077068476d7a7b855889419b292525fb2bc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/574896
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>