feat: Support Solidity endianess with fdp-contracts

[molekilla]

In BMTFile.sol, in the unit tests we add an additional property to ChunkInclusionProof named spanValue, that is the value return by getSpanValue, which is a little endian uint32 value.

The unit test fills this value using BN.js which has support for endianess.

      // TODO: Move this logic to bmt.js
      const proofChunksBigNumber = proofChunks.map((i: { span: Uint8Array; sisterSegments: Uint8Array[] }) => {
        return {
          // eslint-disable-next-line node/no-unsupported-features/es-syntax
          ...i,
          // Note: It has to be exactly little endian with length of 8 bytes
          spanValue: new BN(i.span).toBuffer('le', 8),
        }
      })

The struct in Solidity is:

  struct ChunkInclusionProof{
    // big endian value
    uint64 span;
    // little endian value
    uint64 spanValue;
    bytes32[] sisterSegments;
  }

Ideally, bmt.js must implement spanValue and use BN.js to calculate the endianess, thus the unit test implementation can be removed, making for an easier fdp-contracts API to use for developers.