use MmapStore for MerkleTree generation

[schomatis]

Blocked on filecoin-project/merkletree#13, after that is merged the dependency here should point to merkle_light/master.

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This is now dropping the allocated memory an order of magnitude:

# master:

Total: 309.4 MB
   309.0  99.9%  99.9%    309.0  99.9% ::allocate_in (inline) /rustc/14997d56a550f4aa99fe737593cd2758227afc56/src/liballoc/raw_vec.rs:109
     0.3   0.1% 100.0%      0.3   0.1% ::reserve_internal (inline) /rustc/14997d56a550f4aa99fe737593cd2758227afc56/src/liballoc/raw_vec.rs:683
     0.1   0.0% 100.0%      0.1   0.0% sapling_crypto::jubjub::JubjubBls12::new (inline) ??:0

# This PR with mmap'ed MT:

Total: 19.4 MB
    13.0  67.1%  67.1%     13.0  67.1% ::allocate_in (inline) /rustc/14997d56a550f4aa99fe737593cd2758227afc56/src/liballoc/raw_vec.rs:109
     4.3  22.0%  89.1%      4.3  22.0% ::reserve_internal (inline) /rustc/14997d56a550f4aa99fe737593cd2758227afc56/src/liballoc/raw_vec.rs:683
     2.0  10.3%  99.4%      2.0  10.3% rayon::iter::collect::special_extend@1594e0 (inline) :0

[porcuquine]

This seems fine when the rest is sorted out (might be already).