Make `from_slice()` const?

- Version of tinytga in use (if applicable): 0.5.0

## Description of the problem/feature request/other

I'm experimenting with embedded-graphics on a very restricted system with only 40 kB available RAM, and while it does work with primitives like circles, fonts etc., using tinytga, the size grows up to 80 kB bytes or so while loading a 2 kB file. I therefore wonder if `fn from_slice()` could be made `const` so that image processing is done at compile-time? I realise that this could be an elaborate task due to the limitations of [constant expressions](https://doc.rust-lang.org/reference/const_eval.html).