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- move `psd_tools.api.composer` module to `psd_tools.composer` package;
- support 19 blending modes in composer;
- support fill opacity;
- fix image size when composing with masks;
- rename `TaggedBlockID` to `Tag`;
- rename `ImageResourceID` to `Resource`;
- add `bytes` mixin to `Enum` constants;
- replace `Enum` keys with raw values in `psd_tools.psd.base.Dict` classes.