add mmaped b+ tree (#207)

This PR adds a custom mmaped B+ tree. This data structure creates a mapping from uint64 to uint64.

Structure of node:
Each node in the node is of size pageSize. Two kinds of nodes. Leaf nodes and internal nodes.
Leaf nodes only contain the data. Internal nodes would contain the key and the offset to the child node.
Internal node would have first entry as:
<0 offset to child>, <1000 offset>, <5000 offset>, and so on...
Leaf nodes would just have: <key, value>, <key, value>, and so on...
Last 16 bytes of the node are off limits.
| pageID (8 bytes) | metaBits (1 byte) | 3 free bytes | numKeys (4 bytes) |