Replace compress/zlib with compress/flate for the encoder

[whichxjy]

This PR replaces compress/zlib module with compress/flate. It use the deflate algorithm for encoder directly instead of using the zlib.

Take this PlantUML text for example:

@startuml
Bob -> Alice : hello
@enduml

When the encoder uses compress/zlib, it would be encoded as UDfoA2v9B2efpStXSifFKj2rKt3CoKnELR1Io4ZDoSddSaZDIodDpG44003___W93C00. But https://www.plantuml.com/plantuml/png/UDfoA2v9B2efpStXSifFKj2rKt3CoKnELR1Io4ZDoSddSaZDIodDpG44003___W93C00 is invalid.

When the encoder uses compress/flate, it would be encoded as SYWkIImgAStDuNBAJrBGjLDmpCbCJbMmKiX8pSd9vt98pKifpSq11000__y0. And https://www.plantuml.com/plantuml/png/SYWkIImgAStDuNBAJrBGjLDmpCbCJbMmKiX8pSd9vt98pKifpSq11000__y0 is valid.