context now supports memory only: it takes storage option.

storage arguments are now explicit to support contexts without storages

Benchmark on kurama:

TMPDIR=/mnt/ssd/tmp/ dune exec ./benchmark.exe million

commit 3430b233 Done: 1036146 time 173.98: maxrss delta: 3123360 done: 1036146 time 173.60: maxrss delta: 3125172 done: 1036146 time 173.57: maxrss delta: 3124552 done: 1036146 time 174.08: maxrss delta: 3125360 done: 1036146 time 173.48: maxrss delta: 3124864 done: 1036146 time 173.51: maxrss delta: 3124280

commit 60a22e8c jun@memory_only_context done: 1036146 time 172.34: maxrss delta: 3123976 done: 1036146 time 172.83: maxrss delta: 3123644 done: 1036146 time 172.69: maxrss delta: 3124600 done: 1036146 time 172.36: maxrss delta: 3123708

Somehow jun@memory_only_context is slightly faster, opposite of my expectation.