`MonoFont` character lookup is O(n), when it could be O(1), and mapping '\0' is impossible

[iriswebb]

https://docs.rs/embedded-graphics/latest/src/embedded_graphics/mono_font/mapping.rs.html#139

[rfuest]

The slow linear search has already been fixed in the development version of e-g: #790

Why would you need to map \0?