Support 32 bits

[samoht]

See mirage/mirage-www#562 (comment)

It seems that the recent commits to speed-up hashing broke 32 bits support. We should test this (and fix the issue, probably by removing support for int in Irmin.Type)

[samoht]

@hannesm why do you think these 2 lines might break something? It seems to me that we always try to serialise to 64b, whatever the platform is, which is the right way to have a portable binary serialization. WDYT?

[hannesm]

I said might in another thread since Int64.to_int (actually done in https://github.com/mirage/irmin/pull/458/files#diff-0b43dc6466ffe5ba4e4629f71c33631eR898) sounds very unsafe to me. I don't have sufficient insight into the code base to analyse whether there is an actual problem. Also I'm not sure when this code is executed, and with what input (controlled by whom?), and whether the goals include dumping on 64bit and restoring on 32bit.

On 64bit, the Int64.to_int is also "unsafe": Int64.to_int Int64.max_int -> -1

[samoht]

See #469