Image rework for soundness and usability

[cyrgani]

This PR is a huge rework of the Image type to fix a soundness issue and other bugs, increase performance and improve ergonomics. It also adds deprecation warnings to prepare users for future breaking changes.
Fixes #634.
Fixes #746.
Fixes #637.
Fixes #544.

New methods

• Image::from_raw_parts: method for creating an image from width, height and bytes while checking if the amount of bytes is correct
• Image::bytes, Image::bytes_mut: returns an immutable / mutable slice of all bytes in the image
• Image::width_mut, Image::height_mut, Image::bytes_vec_mut: unsafe methods that return a mutable reference to the respective field, allowing for unchecked manipulation (mostly exist to support migration of strange usecases)
• Image::pixel_amount: returns the amount of pixels this image has according to its dimensions
• Image::filled_with_color: creates an image filled with one color, previously called Image::gen_image_color
• Image::set_image_data: updates all of the image data given a &[[u8; 4]]

Bugfixes

• Reading an out-of-bounds pixel with Image::get_pixel correctly panics now
• Image::blend and Image::overlay panic now if the two images have the same amount of pixels, but different width and height

Other non-breaking changes

• Some Into implementations for Color are now From implementations (should give Into automatically)
• Image::filled_with_color / Image::gen_image_color is 2-3 times faster now
• Image::sub_image is 2 times faster now
• Image::empty, Image::width, Image::height are const fns now (done in make all methods const that can be const on 1.70.0 #850)
• Image::gen_image_color is a deprecated alias of Image::filled_with_color now
• Accessing any of the Image fields directly gives a deprecation warning and suggests using the methods instead.

Planned breaking changes for version 0.5

• Make Image.width, Image.height and Image.bytes private
• Change return type of Image::width and Image::height from usize to u16
• Change argument type of x and y in Image::set_pixel and Image::get_pixel from u32 to u16

[cyrgani]

@not-fl3 I think this is ready for reviewing now.

[cyrgani]

Also fixes #544.