feat: add custom session id generator, based on shadow-tls's impl #6

Job	Run time
BoGo test suite	1m 26s
Build+test (stable, ubuntu-latest)	7m 25s
Build+test (stable, windows-latest)	15m 18s
Build+test (stable, macos-latest)	6m 58s
Features	30s
Build+test (beta, ubuntu-latest)	7m 34s
Build+test (nightly, ubuntu-latest)	7m 12s
Run benchmarks	1m 54s
Check for documentation errors	1m 59s
Measure coverage	8m 44s
cross-target testing (armv7-linux-androideabi)	7m 49s
Format	9s
cross-target testing (i686-linux-android)	9m 6s
Format (unstable)	14s
cross-target testing (thumbv7neon-linux-androideabi)	7m 44s
Check minimum versions of direct dependencies	33s
cross-target testing (i586-unknown-linux-gnu)	3m 36s
Check semver compatibility	52s
cross-target testing (i686-unknown-linux-gnu)	3m 20s
Validate external types appearing in public API	1m 28s
cross-target testing (riscv64gc-unknown-linux-gnu)	6m 52s
cross-target testing (s390x-unknown-linux-gnu)	8m 13s
Clippy	2m 36s
Clippy (Nightly)	2m 31s
cross-target testing (aarch64-linux-android)	3m 54s
Run openssl-tests	1m 25s
Smoke-test fuzzing targets	3m 17s
MSRV	4m 7s
	2h 6m 46s

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