x509roots/fallback: store bundle certs directly in DER

goos: linux
goarch: amd64
pkg: golang.org/x/crypto/x509roots/fallback
cpu: AMD Ryzen 5 4600G with Radeon Graphics
            │ /tmp/before │             /tmp/after              │
            │   sec/op    │   sec/op     vs base                │
InitTime-12   1.726m ± 0%   1.101m ± 1%  -36.20% (p=0.000 n=30)

            │  /tmp/before  │              /tmp/after              │
            │     B/op      │     B/op      vs base                │
InitTime-12   1178.2Ki ± 0%   779.8Ki ± 0%  -33.81% (p=0.000 n=30)

            │ /tmp/before │             /tmp/after             │
            │  allocs/op  │  allocs/op   vs base               │
InitTime-12   11.35k ± 0%   10.64k ± 0%  -6.32% (p=0.000 n=30)

Updates golang/go#73691

Change-Id: Ic33f2fdfc65001c41afeb3b6af8a383288d10de6
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/676217
Reviewed-by: Roland Shoemaker <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>

Author: mateusz834