Replace string-based flags with enums

[francesca64]

Closes #3

This changes all functions that previously accepted a &str flag to instead accept an appropriate enum. There are various changes to note:

• For easy access, these enums are re-exported in the crate root, so anyone who wants to use BlurMode just has to use raster::BlurMode;.
• The transform modes were previously "x" and "y", but I changed them to Horizontal and Vertical, as it's more explicit, expressive, and clear - and thus more rustic. For people with autocompletion, the loss of brevity is a non-issue, though I'd understand if you're not receptive to this change.
• In various places where the possible string flags were enumerated in the documentation, I removed them. Anyone who wants to know all variants just has to click on the enum type in the function signature, and additionally, the example sections showcase the possibilities. In situations where the modes were accompanied by descriptions, those descriptions are now present in the enum's documention, i.e. with ResizeMode. Still, I feel this change may be slightly controversial, which is why I feel the need to mention it here.
• I created a private BlendFunction enum that's just like the public BlendMode enum, except it doesn't have a Normal variant. BlendFunction was made specifically for consumption by ch_alpha_f, whereas BlendMode is part of the public API and used in all other places.