Add non-panicking get_pixel/mut methods to ImageBuffer

[okaneco]

This relands part of #1229 which added methods to ImageBuffer that return
Option<&P> and Option<&mut P> from get_pixel[_mut]-like methods. It does
not include adding methods to the GenericImage and GenericImageView traits.

Naming is still a concern #1243.

[fintelia]

If this is purely about getting the optimizer to produce better code, I wonder if we should instead do something like:

if x >= self.width { 
    return None;
}
let num_channels = <P as Pixel>::CHANNEL_COUNT as usize;
let i = (y as usize).saturating_mul(self.width as usize).saturating_add(x as usize);

self.data.get(i..i+num_channels)
    .map(|pixel_indices| <P as Pixel>::from_slice(pixel_indices))

This way we incorporate the invariant that width * height = data.len(), and avoid an extra check.

[okaneco]

Thanks for that suggestion, I've incorporated that change.

I'm interested in better code generation but more so in providing methods that return Option or Result to users where possible instead of panicking.