Implement Additional Pooling Strategies in FastC

[brunneis]

In addition to Mean Pooling, other basic strategies should be available: Attention Pooling, Max Pooling, and CLS Pooling.

[brunneis]

https://github.com/EveripediaNetwork/fastc/pull/8

• Mean Pooling
• Masked Mean Pooling
• Max Pooling
• Masked Max Pooling
• Attention Weighted Pooling
• Sum Pooling
• CLS Pooling