Make encoder block aware

[creationix]

• [x] Only use pointers if they don't cross block boundaries.
• [ ] Automatically upgrade maps to counted maps if the key-value pairs don't fit in a single block.
• [ ] Automatically upgrade maps to indexed maps if the keys don't fit in a single block.
• [ ] Automatically updated lists to indexed lists if the values don't fit in a single block.

[creationix]

Block aware pointer logic added here