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summary

this PR implements conversion between MouseButton/KeyCode and their underlying primitive types (with tests). this should make it easier to serialize & deserialize mouse/key codes for rebinding game controls, etc.

the specific trait implementations in question are:

• impl From<u8> for MouseButton
• impl From<MouseButton> for u8
• impl From<u16> for KeyCode
• impl From<KeyCode> for u16

implementation notes

• the conversion impls are generated by wrapping the enum definition in a simple declarative macro called convertible_enum!. currently, rust lets you safely convert an enum to its underlying repr, but not vice versa (other than transmute tricks which cause UB on invalid values). to the best of my knowledge, this is the best solution in terms of minimizing boilerplate and not bringing in something like num_enum (which pulls in proc_macro, which is just overkill for this IMO).
• i chose to implement From rather than TryFrom since the enums in question both have an Unknown variant to catch invalid values already. with TryFrom, a successful conversion could still have Unknown as the value, which felt weird and redundant to me.